Prepare Pastry: In medium-size bowl, combine flour and salt. With pastry blender or 2 knives, cut butter and shortening into flour mixture until it resembles fine crumbs. Add water, one tablespoon at a time, and mix lightly with fork until pastry holds together when gently pressed. Shape pastry into 2 balls, wrap in plastic wrap, and refrigerate 30 minutes.
Prepare Filling: Heat broiler. In custard cup, combine garlic, thyme, salt, and pepper. In large bowl, toss zucchini, yellow squash, and eggplant with 1 tablespoon oil and half of garlic mixture. Arrange seasoned vegetables and the yellow pepper quarters in single layer on baking sheet. Broil vegetables 4 inches from heat source until tender but not browned -- about 1 minute. With tongs, remove zucchini, squash, and eggplant to bowl.
Continue to broil pepper quarters, skin side up, until skin wrinkles and browns. With tongs, remove pepper quarters from broiler and carefully place in brown paper bag; twist shut to allow pepper to steam. Set aside 20 minutes or until cool enough to handle.
Meanwhile, in small bowl, with fork, combine goat cheese, water, and remaining garlic mixture; set aside. In small skillet, heat 1 tablespoon of the oil; add onion and sauté until translucent. Set sautéed onion aside in skillet.
Heat oven to 400 degrees F. Between 2 sheets of floured waxed paper, roll out one ball of pastry to an 8 1/2-inch round. Remove top sheet of paper and invert pastry into 7-inch pie pan. Press pastry into bottom and side of pan; remove remaining sheet of waxed paper. Trim off pastry at rim of pan. Repeat with remaining ball of dough.
With slotted spoon, divide onion between pastry-lined pans; reserve any oil remaining in skillet. Divide goat-cheese mixture over onion and top with layers of broiled, sliced vegetables. Place one artichoke half in center of each tart, if desired. Arrange tomato halves spoke fashion on tarts, alternating colors. Drizzle oil from skillet over tarts.
Bake tarts 35 to 40 minutes or until pastry is well browned. Cool tarts on wire rack 10 minutes.
Meanwhile, make yellow-pepper coulis: Peel broiled pepper quarters and place in blender with remaining 1 tablespoon oil and the vinegar. Blend until smooth puree forms.
To serve tarts, loosen edges with spatula. Lift one tart gently with spatula, tipping pan and sliding tart onto serving plate. Repeat with remaining tart. Spoon yellow pepper coulis around tarts and, if desired, garnish with chervil sprigs and chopped red tomato.
