Cut 1 eggplant crosswise into 8 (1/4-inch-thick) round slices. Cut the other eggplant lengthwise into 8 (1/4-inch-thick) oblong slices. Place all of the eggplant slices in a large bowl; pour milk over eggplant slices, and let stand 5 minutes. Drain in a colander, and pat eggplant slices dry with paper towels. Brush 1 tablespoon oil over eggplant slices, and sprinkle the slices with 1/4 teaspoon black pepper and 1/8 teaspoon salt.
Prepare grill. Place eggplant slices on grill rack coated with cooking spray, and grill 3 minutes on each side or until tender. Set aside.
Cut red and yellow bell peppers in half lengthwise; discard seeds and membranes. Place the peppers, skin sides up, on a foil-lined baking sheet; flatten with hand. Broil 15 minutes or until blackened. Place the bell peppers in a zip-top heavy-duty plastic bag, and seal; let stand 10 minutes. Peel and discard skins. Coarsely chop bell peppers, and set aside.
Bring 3 quarts water to a boil in a large Dutch oven. Add spinach, and cook 30 seconds. Drain well, and let cool. Squeeze spinach until barely moist; set aside.
Heat 1 teaspoon oil in a saucepan over medium heat. Add shallots and garlic, and saute 2 minutes. Add 1 cup chopped tomato, chopped basil, 1/4 teaspoon black pepper, 1/8 teaspoon salt, and sugar, and cook 10 minutes, stirring occasionally.
Place 1 round eggplant slice in the bottom of each of 4 (10-ounce) ramekins coated with cooking spray. Line sides of each ramekin with 2 oblong eggplant slices. Layer roasted red bell pepper, spinach, tomato mixture, and roasted yellow bell pepper evenly among ramekins; top with remaining round eggplant slices, pressing firmly to pack. Place ramekins on a baking sheet; bake at 250° for 20 minutes. Invert ramekins onto individual plates. Spoon Yellow Bell Pepper Coulis and Red Bell Pepper Coulis alternately around each gâteau, using 1 tablespoon for each "pool" of sauce; top each with 1 teaspoon chopped tomato and a basil sprig.