In a large skillet, heat olive oil and cook the onions until soft (don't brown).Spoon the onions into the crockpot, and add the tomatoes, water (if using) and spices. Mix thoroughly.Cover and cook on low all day, or overnight (I do mine for about 10 hours) thenuse a blender or immersion stick blender to make it smooth.Serve over pasta, or freeze in 2 cup portions, or can in jars