Preheat the oven to 350 degrees F (175 degrees C). Grease a 9x13 inch baking dish.
Melt the margarine in a large skillet over medium heat. Add onions; cook and stir until tender. Turn off the heat, and stir in the sour cream and Parmesan cheese. Transfer to the prepared baking dish, and crumble the crackers over the top.
Bake uncovered for 30 minutes in the preheated oven, until the top is browned and the sauce is bubbly.
