1. Preheat oven to 450°.
2. Place heirloom tomato slices on several layers of paper towels. Top with a single layer of paper towels. Let stand 15 minutes.
3. Combine 1 cup water, garlic, leek, and quartered tomato in a medium saucepan; bring to a boil. Reduce heat; simmer 10 minutes. Strain through a sieve over a bowl, pressing to extract liquid; discard solids. Pour liquid into pan; stir in 1/2 teaspoon salt and 1/4 teaspoon pepper. Cool to room temperature. Sprinkle with gelatin; let stand 5 minutes. Bring gelatin mixture to a boil; boil 3 minutes. Remove from heat.
4. Lightly brush eggplant slices with oil; arrange in a single layer on a baking sheet coated with parchment paper. Bake at 450° for 10 minutes; turn and bake an additional 5 minutes or until tender.
5. Lightly coat a 9 x 5-inch loaf pan with cooking spray. Line pan with plastic wrap. Arrange tomato slices in a double layer on bottom of pan; sprinkle with remaining 1/4 teaspoon salt and remaining 1/4 teaspoon pepper. Top tomatoes with a thin layer of cheese; sprinkle with 1 1/2 teaspoons thyme and 1 1/2 teaspoons chives. Arrange 3 eggplant slices over cheese. Repeat layers, ending with tomatoes. Pour gelatin mixture over terrine. Cover with plastic wrap. Chill 8 hours or overnight.
6. Turn terrine out onto a platter. Let stand 1 hour (or until room temperature).