In a large skillet, heat 2 tablespoons of the olive oil. Add the tomatoes, 1 garlic clove and 1/4 teaspoon of the crushed red pepper and season with salt. Cook the tomatoes over moderate heat, stirring occasionally, until just softened, about 5 minutes. Scrape the tomatoes into a medium saucepan and discard the garlic clove. Wipe out the skillet. Repeat with the eggplant, zucchini, onions and red bell pepper, cooking each vegetable separately in 2 tablespoons of oil with 1 garlic clove, 1/4 teaspoon of crushed red pepper and a generous pinch of salt until just tender and lightly browned, about 7 minutes per vegetable. Add the cooked vegetables to the tomatoes in the saucepan.