1) Unroll pastry sheets into two 9-in. fluted tart pans with removable bottoms; trim edge. Refrigerate 30 minutes. Preheat oven to 400°.
2) Line unpricked pastry with a double thickness of foil. Fill with pie weights, dried beans or uncooked rice. Bake on a lower oven rack 10-15 minutes or until edges are golden brown. Remove foil and weights; bake 2-4 minutes longer or until bottom is golden brown. Cool on a wire rack. Reduce oven setting to 375°.
3) In a large skillet, heat oil over medium-high heat. Add onion; cook and stir 5-7 minutes or until tender and lightly browned. Remove from pan. Add butter and mushrooms; cook and stir 6-8 minutes or until mushrooms are tender. Cool slightly4) Place tart pans on separate baking sheets. Divide onion and mushrooms between crusts; top with salmon and cheese. In a bowl, whisk 4 eggs, 2 teaspoons capers and 1/4 teaspoon each salt and pepper; if desired, stir in 1 teaspoon dill. Pour over one of the tarts. Repeat with remaining ingredients.
Bake 15-20 minutes or until a knife inserted near the center comes out clean. Let stand 5 minutes before cutting.
Yield: 2 tarts (6 servings each; 12 servings total).
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