1. Heat a large skillet over medium heat. Add 1 tablespoon oil to pan; swirl to coat. Add onion; cook 6 minutes, stirring occasionally. Stir in tomato paste; cook 2 minutes. Stir in rind, thyme, 1/8 teaspoon salt, and pepper; cook 2 minutes, stirring occasionally.
2. Preheat broiler to high.
3. Arrange zucchini on a jelly-roll pan. Drizzle with remaining 1 tablespoon oil; toss. Broil 7 minutes or until lightly charred. Sprinkle with remaining 1/4 teaspoon salt.
4. Preheat oven to 375°.
5. Spread onion mixture in a 2-quart gratin dish. Arrange zucchini mixture over onion mixture. Sprinkle with cheese. Cover and bake at 375° for 25 minutes. Remove from oven.
6. Preheat broiler to high.
7. Uncover zucchini mixture; broil 1 1/2 minutes or until lightly browned.