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1
Sprinkle the yeast and sugar over 1/2 cup of warm water in a large bowl.
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2
The water should be no more than 100 degrees F (40 degrees C).
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3
Let stand for 5 minutes until the yeast softens and begins to form a creamy foam.
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4
Beat in 1 egg and butter.
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5
Gently stir in the flour, salt, and lemon zest until the dough has pulled together.
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6
Gather the dough into a ball, then place in a lightly oiled bowl.
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7
Turn dough ball to coat with oil.
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8
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.
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9
Punch down the dough, and place in a 10-inch tart pan with a removable bottom.
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10
Spread dough out from the center using fingers.
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11
The dough should be thinner in the center and thicker at the edges.
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12
Chill in the refrigerator for 20 minutes.
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13
Preheat an oven to 350 degrees F (175 degrees C).
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14
Heat the vegetable oil in a skillet over medium heat.
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15
Stir in the onion; cook and stir until the onion has softened and turned translucent, about 5 minutes.
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16
Sprinkle chilled tart shell with Gouda and Parmesan cheeses.
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17
Arrange the cooked onion and piquillo peppers decoratively over the cheese.
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18
Beat 2 eggs and heavy cream with the mustard in a bowl; pour egg mixture over the onion and cheese.
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19
Bake in the preheated oven until the crust is golden brown and the custard has set, 30 to 45 minutes.
