FILLING 2 tablespoons olive oil 1 large onion, halved, thinly sliced 1 tablespoon red wine vinegar 1 ripe pear, peeled, halved, cored 1 tablespoon unsalted butter 2 ozs. Roquefort or blue cheese, crumbled 3 tablespoons chopped toasted walnuts 2 teaspoons walnut oil FoR CRUST: Combine flour and butter in processor. Using on/off turns, pulse until mixture resembles coarse meal. Mix water and egg yolk in small bowl. Pour mixture into processor through feed tube. Using on/off turns, pulse until moist clumps form (do not overmix). Gather dough together. Roll out on lightly floured surface to 14-inch square. Cut into four 6-inch rounds. Line four 4-inch-diameter tartlet pans with removable bottoms with dough. Trim edges. Cover crusts and chill 30 minutes. FOR FILLING: Heat olive oil in heavy medium skillet over medium heat. Add thinly sliced onion and saute until golden, about 25 minutes. Mix in red wine vinegar. Season with salt and pepper. Saute 1 minute. (Crusts and onions can be prepared 1 day ahead. Keep crusts chilled. Cover onions and refrigerate.) Preheat oven to 350° F. Line crusts with foil and fill with dried beans or pie weights. Bake until crusts are set, about 10 minutes. Remove foil and beans. Bake crusts until light brown, about 12 minutes. Cool. Cut each pear half into 6 wedges. Melt 1 tablespoon butter in heavy medium skillet over medium heat. Add pear wedges; saute until golden, about 2 minutes. Season with salt and pepper. Cool. (Can be made 3 hours ahead. Cover crusts and pears separately.) Preheat oven to 375° F. Divide sauteed onion among crusts. Top with cheese. Place 3 pear wedges in each. Sprinkle with walnuts and drizzle with walnut oil. Bake until heated through, about 8 minutes. Serve tartlets warm. Makes 4. pc 141b