In a small bowl: mix yeast and warm water and let stand until foamy, about 5 minutes.
Put the flour in a medium bowl, then make a well in center of flour and add yeast. Stir together egg, 1 tablespoon oil, and 1 1/2 teaspoons salt. Add egg mixture to yeast mixture and mix with a wooden spoon gradually incorporating flour, until a soft dough forms. Transfer dough to a floured surface and knead, working in additional flour (up to 1/4 cup) as necessary, until smooth and elastic, about 5 minutes. Transfer dough to an oiled bowl and turn to coat. Cover with plastic wrap and let rise in a draft-free place at warm room temperature until doubled, 1 to 1 1/2 hours.
While dough rises, heat remaining 1/3 cup oil in a heavy skillet or similar over medium-high heat until it shimmers, then lightly sauté fennel seeds, about 30 seconds. Stir in onions, remaining teaspoon salt, and 1/2 teaspoon pepper, then reduce heat to medium-low and cover onions directly with a round of parchment paper. Cook, stirring occasionally, until onions are very tender and golden brown, 30 minutes to 1 hour.
Take of the heat and let cool down
Preheat oven to 375°F.
Knead dough gently on a floured surface into a 15- by 12-inch rectangle, turning up or crimping edge, then brush mustard evenly over dough, leaving a 1/2-inch border around edge. Spread onions evenly over mustard, then sprinkle evenly with cheese.
Bake tart until crust is golden brown, 30 to 35 minutes. Cut into 2-inch squares or diamonds and serve warm or at room temperature.
Thanks to smitten kitchen for this discovery!
You’ll need:
In a small bowl: mix yeast and warm water and let stand until foamy, about 5 minutes.
Put the flour in a medium bowl, then make a well in center of flour and add yeast. Stir together egg, 1 tablespoon oil, and 1 1/2 teaspoons salt. Add egg mixture to yeast mixture and mix with a wooden spoon gradually incorporating flour, until a soft dough forms. Transfer dough to a floured surface and knead, working in additional flour (up to 1/4 cup) as necessary, until smooth and elastic, about 5 minutes. Transfer dough to an oiled bowl and turn to coat. Cover with plastic wrap and let rise in a draft-free place at warm room temperature until doubled, 1 to 1 1/2 hours.
While dough rises, heat remaining 1/3 cup oil in a heavy skillet or similar over medium-high heat until it shimmers, then lightly sauté fennel seeds, about 30 seconds. Stir in onions, remaining teaspoon salt, and 1/2 teaspoon pepper, then reduce heat to medium-low and cover onions directly with a round of parchment paper. Cook, stirring occasionally, until onions are very tender and golden brown, 30 minutes to 1 hour.
Take of the heat and let cool down
Preheat oven to 375°F.
Knead dough gently on a floured surface into a 15- by 12-inch rectangle, turning up or crimping edge, then brush mustard evenly over dough, leaving a 1/2-inch border around edge. Spread onions evenly over mustard, then sprinkle evenly with cheese.
Bake tart until crust is golden brown, 30 to 35 minutes. Cut into 2-inch squares or diamonds and serve warm or at room temperature.