Sautee the onion in 2 tablespoons of the butter on low heat, until golden brown and very soft. Stir the onion mixture into the cracker crumbs. Melt an additional 2 tablespoons of the butter and add to the crumbs. Stir until well mixed. The crumbs should stick together somewhat when pressed with a fork or spoon. Take one tablespoon of the butter and grease a shallow casserole dish or baking pan.
If you are using thin fillets: Lay out each fillet on a plate and spread a small amount of the cracker stuffing along its length. Press it down so that it will stay in place. Roll the fillet up and place in the buttered casserole dish or baking pan (seam down). Repeat with all the fillets. Melt the remaining 2 tablespoons of butter and drizzle over or baste the tops of the rolls.
If you are using thick fillets: Place the fish in the buttered casserole dish or baking pan. Spoon the cracker stuffing onto each piece and spread it out, pressing it down so that it will stay in place. Melt the remaining tablespoon of butter and drizzle over the fillets.
At this point you can cook the fish or cover and place in the refrigerator up to several hours.
To cook the fish: Preheat the oven to 425 degrees. Place the baking dish in the top third of the oven and bake for 10 minutes. The rolls should be done at this point, but the thicker fillets may need an extra few minutes. Use a fork to test the center of the thickest piece for doneness. Thinner pieces will cook more quickly. Serve immediately.