Leek and Onion Tart Recipe

• 2 1/4 teaspoons active dry yeast (a 1/4-oz package)
• 1/2 cup warm water (105–115°F)
• 1 1/2 to 1 3/4 cups all-purpose flour
• 1 large egg
• 1/3 cup plus 1 tablespoon extra-virgin olive oil, divided
• 2 1/2 teaspoons salt, divided
• 2 teaspoons fennel seeds
• 1 pound leeks, cleaned, halved and thinly sliced
• 2 pounds yellow onions, halved and thinly sliced
• 1 tablespoon Dijon mustard
• 1/2 cup Parmigiano-Reggiano, grated

1. Stir together yeast and warm water in a small bowl and let stand until foamy, about 5 minutes. (If mixture doesn’t foam, add 1 teaspoon of sugar.)
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. Add egg mixture to yeast mixture and mix with a wooden spoon or your fingertips, 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.
3. 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 leeks and 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 leeks and onions are very tender and golden brown, 1 to 1 1/4 hours.
4. Preheat oven to 375°F with rack in middle.
5. 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. Spread leeks and onions evenly over mustard, then sprinkle evenly with cheese.
6. 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.

Makes 4 servings or 8 servings as an hors d'oeuvre

Gourmet Note: Onion mixture can be made 2 days ahead and chilled, covered.

[Adapted from Gourmet magazine]