RED ONION TART

Steps:

• Sprinkle the yeast and sugar over 1/2 cup of warm water in a large bowl. The water should be no more than 100 degrees F (40 degrees C). Let stand for 5 minutes until the yeast softens and begins to form a creamy foam. Beat in 1 egg and butter. Gently stir in the flour, salt, and lemon zest until the dough has pulled together. Gather the dough into a ball, then place in a lightly oiled bowl. Turn dough ball to coat with oil. Cover with a light cloth and let rise in a warm place (80 to 95 degrees F (27 to 35 degrees C)) until doubled in volume, about 45 minutes.
• Punch down the dough, and place in a 10-inch tart pan with a removable bottom. Spread dough out from the center using fingers. The dough should be thinner in the center and thicker at the edges. Chill in the refrigerator for 20 minutes.
• Preheat an oven to 350 degrees F (175 degrees C).
• Heat the vegetable oil in a skillet over medium heat. Stir in the onion; cook and stir until the onion has softened and turned translucent, about 5 minutes. Sprinkle chilled tart shell with Gouda and Parmesan cheeses. Arrange the cooked onion and piquillo peppers decoratively over the cheese. Beat 2 eggs and heavy cream with the mustard in a bowl; pour egg mixture over the onion and cheese.
• Bake in the preheated oven until the crust is golden brown and the custard has set, 30 to 45 minutes.