Onion Tart with Mustard and Fennel

Ingredients

• 2.3 teaspoons yeast dry (a)
• 1 tablespoon dijon mustard 
• 1 large eggs 
• 2 teaspoons fennel seeds 
• 1.5 cups flour all-purpose
• 0.3 cup olive oil extra virgin extra-virgin divided
• 0.5 cup parmesan grated
• 2.5 teaspoons salt divided
• 0.5 cup water (105-115°F)
• 3 pound onion yellow halved thinly sliced

Equipment

• bowl
• frying pan
• baking sheet
• baking paper
• oven
• plastic wrap
• wooden spoon

Directions

1. Stir together yeast and warm water in a small bowl and let stand until foamy, about 5 minutes. (If mixture doesn't foam, start over with new yeast.)
2. Put 1 1/2 cups flour in a medium bowl, then make a well in center of flour and add yeast mixture to well. Stir together egg, 1 tablespoon oil, and 1 1/2 teaspoons salt with a fork.
3. Add egg mixture to yeast mixture and mix with a wooden spoon or your fingertips, gradually incorporating flour, until a soft dough forms.
4. 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.
5. 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.
6. While dough rises, heat remaining 1/3 cup oil in a 12-inch heavy skillet over medium-high heat until it shimmers, then sauté fennel seeds until a shade darker, 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, 1 to 1 1/4 hours.
7. Preheat oven to 375°F with rack in middle.
8. Knead dough gently on a floured surface with floured hands to deflate. Pat out dough on a large heavy baking sheet (preferably blue steel) 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.
9. Spread onions evenly over mustard, then sprinkle evenly with cheese.
10. Bake tart until crust is golden brown, 30 to 35 minutes.
11. Cut into 2-inch squares or diamonds and serve warm or at room temperature.
12. Onion mixture can be made 2 days ahead and chilled, covered.