Preheat the oven to 300 degrees F. Line a 9 by 13 by 2-inch casserole dish with aluminum foil. Sprinkle the bass with salt and arrange the fillets in the casserole dish.
Make the basting sauce by mixing the ketchup, mayonnaise, mustard, Worcestershire sauce, brown sugar, vinegar, butter, and onion. Pour the sauce over the catfish and bake until the fish flakes easily with a fork, about 20 to 25 minutes. Baste the fish, with the sauce in the pan, every 10 minutes as it bakes. Remove the fish from the oven and using pancake turners, carefully transfer the fish to a serving platter. Garnish with lemon slices and serve.