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Serves 8 as an appetizer or 4 as a main course
1 ½ cups (7 ½ ounces) all-purpose flour
1/2 tsp. salt
4 Tbsp. vegetable shortening, chilled in the refrigerator 30 minutes
4 Tbsp. unsalted butter, cut into ¼ inch pieces, chilled in freezer 30 minutes
2-6 Tbsp. ice water
3 1/2 cups thinly sliced leeks (white and pale green parts only; about 2 medium)
3 Tbsp. extra-virgin olive oil
1 large red onion, thinly sliced
1 cup sliced green onions
1 large egg, beaten to blend
8 ounces Taleggio cheese, cut into small pieces
2 Tbsp. grated Parmesan cheese
Pulse flour and salt in food processor fitted with steel blade until combines. Add shortening and butter and process until mixture rides up the sides of the bowl, and resembles course crumbs. Turn mixture into medium bowl.
Sprinkle water bit by bit over mixture, evenly combine water and flour mixture. Dough should feel wet. Form dough into cohesive ball and wrap in plastic wrap. Refrigerate at least one hour, or up to 2 days, before rolling.
Roll onto lightly floured work surface. Transfer to tart pan.
Combine leeks and oil in large nonstick skillet. Cover and cook over medium-low heat until leeks are tender but not brown, stirring frequently, about 15 minutes. Stir in red onion and green onions. Sauté uncovered until all onions are very tender, about 25 minutes longer. Season with salt and pepper. Cool. Mix in egg, then Taleggio cheese.
Preheat oven to 375°F. Roll out dough on floured surface, forming 13-inch round. Transfer to large rimless baking sheet. Fold outer 1 inch of dough over, forming double-thick rim. Spread topping evenly over crust.
Bake tart 10 minutes. Sprinkle Parmesan over. Bake until crust is golden, about 15 minutes longer. Cut into wedges.