Everywhere I go I see Portuguese tarts. They nestle on a plate beside a bottle of port and a message of welcome in my room at the Sheraton Porto Hotel and Spa. They crowd the glass cake cabinet at Café Majestic on Porto's Rua Santa Catarina. They appear on the table before me, night after night, once the main course has been served and dessert called for. They tempt me, devilishly, irresistibly, so that eventually I begin to resemble a Portuguese tart myself.

"Why do we have such a sweet tooth?" Elisabete Moura Cruz asks, echoing my curiosity as we stroll along the streets of Porto.

"Because of the monks and nuns. They passed it on to us," she concludes.

Cruz, a tour guide, is telling me about this traditional Portuguese dessert that appears to be stalking me as I explore Porto and the nearby Douro Valley, where Scenic is about to launch a new river cruise. I'm an easy target, for I possess a sweet tooth myself and find that there is almost nothing as tempting as this dainty tart, correctly named leite-crème queimado in its country of origin: cups of buttery pastry filled with silky custard and glazed with sugar. Essentially a crème brûlée, the custard is often served sans the crust, as a pudding. No matter; I'm willing to sample any of its variations.

Leite-crème queimado, Cruz explains, is a wicked dessert with pious beginnings: when pilgrims gathered in Portugal's monasteries and convents in the 17th and 18th centuries, they would bring with them gifts of fresh eggs, whose whites would be used to starch the nuns' and abbots' vestments.

But what to do with all those leftover yolks? Looking around for inspiration, the nuns noted that sugar was plentiful (Portuguese explorers had, after all, ferried sugarcane seedlings to the New World, including its colony, Brazil); cinnamon had been traded from India; and there was milk aplenty from the surrounding farms. And so the pilgrims' humble gift of eggs was transformed into leite-crème queimado, and served to them as a warm and delicious welcome.

The sweetness turned sour in 1834, when Portugal's government dissolved male religious orders, banned nuns from taking in novices, and confiscated their properties. But recipes were saved and smuggled secretly from the cloisters on scraps of paper or inside the nuns' owns heads. They contained instructions for the making of leite-creme queimado, of flaky almond tart, of touchino do ceu, a sweet that is conjured from a mix of pumpkin, sugar, almonds and cinnamon and whose name means 'lard from heaven'.

But everything is different now. Catholicism has been reinstated (85 per cent of Portuguese are Catholics, says Cruz; the remaining 15 per cent are bad Catholics); cathedrals have been re-sanctified, monasterial ruins brought back to life. And the Portuguese have continued to uphold the monks' and nuns' hospitable tradition of dessert-offering.

Outside the Our Lady of Remedies, a magnificent baroque church on a hillside in Lemego, a man offers me Rebucados de Regua, a sweet made from honey, lemon and orange and wrapped in twists of paper. At a bakery that sits in the village hundreds of steps below the church, Cruz buys me lemon syrup cake. On a Douro Valley hilltop at the Quinta Nova winery I eat meringue that swims in a pool of strawberry soup. And at Morgadio da Calçada manor house in the village of Provisende, I'm presented with a great big dish of post-dinner leite-creme queimado. It's too much to consume in one sitting, and I find the leftovers on the breakfast table next morning. And so I eat them, too.

Catherine Marshall was a guest of Scenic.

FIVE MORE TREATS

Regional dishes will be served on the new Scenic Azure, which starts cruising through the Douro Valley in April – and onshore excursions will consolidate the local culinary experience.

PORT

Porto is named for the port produced in this region, of course, and the grapes from which this sweet tipple originates can be found growing up and down the valleys of the Douro. Port, wine and sparkling wine varietals are served on-board, at the many quintas visited along the way and at wine bars and restaurants in Porto. The Douro Museum in Peso da Regua takes visitors on an interactive journey through the history of port and winemaking in the region.

OLIVES

Visit the remote Casa Painova estate in the Coa Valley for some of Portugal's most delicious olive products: tapenades, oil, jams, and olives, of course.

ALMONDS

Almond trees flourish across the Douro Valley and the nuts are used in many Portuguese dishes. Scenic guests can take part in a sweet almond workshop to see how they are harvested and transformed into desserts.

BREAD

A Portuguese meal is not complete without bread, and traditional, wood–fired loaves are baked fresh each day at the 200-year-old Bakery Fatima in the village of Provezende. They're best paired with preserves made from fruit grown in the town's tiny, stone-walled orchards.

FISH

Fish is a staple of the Portuguese diet, and fishermen take off early each morning in traditional wooden fishing boats, returning with a catch of sardines, codfish, squid and brill. At a local tavern in the riverside village of Afurada, they will cook some of their catch over a traditional open fire and offer it to guests along with a glass of port wine.

TRIPNOTES

MORE INFORMATION

www.visitportugal.com

GETTING THERE

Etihad Airways flies to Porto from Sydney, Melbourne, Brisbane and Perth via Abu Dhabi and Milan, codesharing with TAP Portugal. Return fares start at $A1916 including taxes. See etihad.com (www.etihad.com)

STAYING THERE

Scenic's 11 Day Unforgettable Douro is a round trip journey from the World Heritage-listed city of Porto to Vega de Terron, across the Spanish border from Portugal. There is no overnight sailing, with the new Scenic Azure taking in vineyards, historic towns and villages as it cruises along the Douro River. Prices start at $6775 per person, twin share. Book now to travel in 2017 at 2016 prices. See swww.scenic.com.au.