Pulse flour,butter,and 1/8 teaspoon salt in a food processor 12 times or until mixture is crumbly. Add 5 tablespoons water; process just until dough begins to leave sides of the bowl and forms a ball.
Divide dough in half; shape each portion into a ball. Cover and chill 20 minutes. Roll each portion of dough to 1/8 inch thickness on a lightly floured surface. Cut pastry into 12 rounds, using a 4 1/2 inch biscuit cutter.
Place rounds on 2 parchment paper lined 15x10 inch jellyroll pans. Crimp edges, if desired. Spread rounds evenly with dijon mustard; top with cheese slices, tomato, thyme, salt and pepper. Sprinkle rounds with shredded cheese; drizzle with oil. Bake at 425 for 15 minutes or until pastry is golden. Garnish, if desired. Serve immediately. Makes 1 dozen tarts.