1. Combine flour, salt, and 1 teaspoon pepper in a food processor. Gradually add butter, pulsing until a coarse meal forms. Add 1 egg yolk and 2 tablespoons water; process until dough sticks together when pressed. Press dough into a 9-inch square or round tart pan. Pierce dough all over with fork; brush with remaining egg yolk, and chill 1 hour.
2. Preheat oven to 375°. Bake crust 15 minutes; set aside.
3. Steam or microwave zucchini 3 to 5 minutes or until tender but still holding its shape. Drain on paper towels.
4. Combine remaining 1/2 teaspoon pepper, mayonnaise, and next 3 ingredients in a bowl. Layer half of zucchini, half of tomato slices, and half of mayonnaise mixture in reserved crust. Repeat with remaining zucchini, tomatoes, and mayonnaise mixture.
5. Bake tart 25 minutes or until filling is hot and bubbly and crust is golden. Garnish, if desired.