Peel, quarter, and slice the onions. Put into a covered, microwave-safe dish and cook on 75% power until tender, about 10 minutes. In a 9 x 13-inch baking dish, layer the onions and cheese. Mix the crushed crackers and melted butter. Sprinkle over the layers of onions and cheese. Pour the milk over it all, and bake at 350 degrees F. for 20-30 minutes.