PreparationMake pastry dough: Blend together flour, butter, and salt in a bowl with your fingertips or a pastry blender (or pulse in a food processor) just until mixture resembles coarse meal with some roughly pea-size butter lumps. Drizzle 5 tablespoons ice water evenly over mixture and gently stir with a fork (or pulse) until incorporated. Squeeze a small handful of dough: If dough doesn't hold together, add more ice water, 1/2 tablespoon at a time, stirring until incorporated. Do not overwork dough, or pastry will be tough. Turn out dough onto a work surface and divide into 4 portions. With heel of your hand, smear each portion once or twice in a forward motion to help distribute fat. Gather all of dough together, with a pastry scraper if you have one, and form into 2 (5-inch) squares. Chill, wrapped in plastic wrap, until firm, at least 1 hour. Make tart shells: Arrange flan rings on 2 parchment-paper-lined baking sheets. Roll out 1 portion of dough on a lightly floured surface with a lightly floured rolling pin into a 16- by 10-inch rectangle. Cut into 6 rough squares. Gently fit each square into a ring (do not stretch dough). Trim excess dough flush with rim. Lightly prick each shell several times with a fork and chill until firm, about 30 minutes. Repeat with remaining dough and rings. Preheat oven to 375°F with racks in upper and lower thirds. Line shells with foil (not heavy-duty) and fill with pie weights. Bake until sides are set, about 20 minutes. Carefully remove weights and foil and bake shells until golden-brown all over, 5 to 8 minutes. Cool completely on baking sheets on racks, then remove flan rings. Reduce oven temperature to 350°F. Assemble goat cheese and tomato tarts: Cook haricots verts in salted boiling water until crisp-tender, about 3 minutes. Drain and divide among 4 tart shells along with tomatoes, then top with a round of cheese. Assemble crab and tarragon tarts: Cook shallot in 1/2 tablespoon oil in a small skillet over medium heat until softened, about 1 minute. Transfer to a small bowl. Heat remaining tablespoon oil in skillet over medium heat, then cook bread, stirring, until golden. Toss croutons with shallot and divide among 4 tart shells. Toss crab with tarragon, zest, and cayenne and add to same shells. Assemble pea, scallion, and pancetta tarts: Cook scallions in oil in cleaned small skillet over medium heat until softened, about 1 minute. Stir in peas and warm through, then divide mixture among remaining 4 tart shells. Add pancetta to skillet and cook, turning once, until just crisp. Reserve pancetta. Make custard and bake tarts: Whisk together custard ingredients with 3/4 teaspoon salt and 1/2 teaspoon pepper. Divide among shells. Sprinkle chives over goat cheese tarts and top pea tarts with pancetta. Bake tarts on baking sheets until custard is just set, about 20 minutes. Cool slightly. Cooks' notes:•Tart shells can be baked 1 day ahead and kept on trays at room temperature, wrapped tightly in plastic wrap.•Filled baked tarts can be kept at cool room temperature 2 hours.•To make 1 large (11-inch) tart, prepare a half recipe of dough and, when fitting it into the tart pan, cut off excess dough, leaving a 1/2-inch overhang, and fold overhang inward. Press dough against side of pan, pushing dough 1/4 inch above rim. Choose 1 filling and triple the ingredient quantities for the filling (for a large pea tart, you’ll need only 5 slices pancetta); use the entire custard recipe. Bake shell as above. Bake filled tart 30 to 35 minutes.