In a saucepan, cook the onions and garlic in oil for about 7 minutes on medium heat, stirring frequently.
Core and cut the cauliflower into florets; to make about 7 cups.
Chop the tomatoes or squeeze them by hand and add them with their juice to the onions. Stir in the raisins and vinegar. When the onions are translucent, add the cauliflower florets and cook for 10 to 12 minutes, or until the florets can be pierced with a fork but are not falling apart.
Add sugar, salt, and pepper, to taste. Serve hot, at room temperature, or chilled.