In a medium mixing bowl stir together flour and sugar. Using a pastry blender, cut in the butter till pieces are the size of small peas. In a small mixing bowl combine the egg yolk, lemon juice, and poppy seed. Gradually stir egg yolk mixture into the flour mixture. Dough will not be completely moistened. Using your fingers, gently knead the dough just till a ball forms. Use the dough immediately, or wrap it in plastic wrap and chill for up to 48 hours. (If chilling, bring the dough to room temperature before using.) Makes enough dough for four 4 to 4-1/2-inch tarts or one 9-inch tart (8 servings). Nutrition facts per serving: 90 cal., 2 g total fat (1 g sat. fat), 28 mg chol., 1 mg sodium, 17 g carbo., 0 g fiber, 2 g pro. PC151