Preheat oven to 425 degrees with a 12-by-14-inch roasting pan on center rack. Rub olive oil over onions and garlic, and place in pan. Season with salt and pepper. Cook until onions begin to brown, 35 to 45 minutes, stirring occasionally. If the pan gets too dry and onions begin to stick, add a few tablespoons of water. Lower heat to 400 degrees. Add raisins, stir, and cook 8 minutes more. Add tomatoes and brown sugar; cook until tomatoes begin to split, 5 to 7 minutes.
Transfer the pan to top of stove over medium-high heat. Add Marsala, and season with salt and pepper. Stir well, scraping up any cooked-on bits. Stir until Marsala has been fully incorporated. Serve the vegetables warm.