1. Preheat oven to 425°.
2. Place eggplant, bell pepper, and onion in a single layer on a large jelly-roll pan. Drizzle vegetables with oil; sprinkle with salt and pepper.
3. Remove white papery skin from garlic head (do not peel or separate the cloves). Cut off pointed end of garlic head 1/4 inch from top. Wrap garlic in foil, and place on pan. Bake at 425° for 30 minutes. (Do not stir vegetables.)
4. Stir tomato into vegetables, and bake an additional 10 minutes or until vegetables are beginning to brown. Cool in pan on a wire rack 10 minutes.
5. Unwrap garlic. Separate cloves; squeeze to extract pulp into a food processor. Discard skins. Add roasted vegetables, lemon rind, and next 3 ingredients to garlic pulp; pulse 12 times or until chunky, scraping sides of bowl once. Set aside.
6. While vegetables bake, heat a cast-iron grill pan over high heat 4 minutes. While pan heats, pat polenta slices dry with paper towels; coat both sides with cooking spray. Coat pan with cooking spray. Add polenta slices, in batches, to pan; cook 5 minutes on each side or until thoroughly heated and scored with grill marks. Remove from pan; keep warm.
7. Spoon 1 1/2 tablespoons salsa over each polenta cake, reserving remaining salsa for another use. Serve warm.