Sprinkle yeast and sugar over 1/4 cup/60 mL lukewarm water; let stand until foamy, about 10 minutes.

In bowl of stand mixer, whisk together flour and 1 tsp/5 mL salt; on slow speed with dough hook, work in yeast mixture and 1-1/2 cups/375 mL lukewarm water. Beat until smooth; beat in yolks until thoroughly mixed.

A bit at a time, beat in butter. Increase speed to medium; beat until dough is smooth and elastic but quite soft, 3 to 5 minutes. Scrape down side of bowl. Seal bowl with plastic wrap and cover with tea towel; in warm place, let dough rise until doubled in volume, about 2 hours.

Meanwhile, in skillet, heat 1/3 cup/75 mL of the oil over medium heat; add onions, bay leaves and pinch of salt and fry, stirring often, until onions are softened and just beginning to colour. Stir in wine and cook until wine has evaporated. Remove from heat; let cool. Remove bay leaves.

Spread bottom of 13- x 10-inch/33 x 25 cm rimmed baking sheet with 2 tbsp/30 mL of the oil. Generously oil work surface and hands. Push down dough, gently working into ball. With hands, scoop out about two-thirds of the dough. Press dough, flattening and stretching to cover entire baking sheet with even thickness; let rest for a few minutes, then stretch again to cover entire sheet.

Leaving about 1-inch/2.5 cm border around edge, spoon onion mixture evenly over dough; lay salmon slices over top.

With oiled hands on oiled work surface, flatten and stretch remaining dough into 11 x 8-inch/28 x 20 cm rectangle; place over filling. Roll rim of bottom dough over edge of top dough, pinching together to seal all around edge. Pat to even thickness all over; spread 2 tbsp/30 mL of the oil over top. With tip of knife, prick 3 or 4 small vents.

Bake in centre of 400 F/200 C convection or conventional oven until golden brown on top and bottom, 30 to 40 minutes. Let cool on pan on rack. Cut into squares.

Makes about 12 servings.