In a microwave-safe bowl, combine butter and onions; microwave on high for 2 minutes.
To the butter mixture, add soft bread crumbs, Parmesan cheese, dried basil, salt and pepper, garlic powder and sugar. Stir.
Spray a 1-quart baking dish with nonstick cooking spray.
Layer 1/3 tomato slices, top with 1/3 bread crumb mixture. Repeat until you have three layers, ending with bread crumbs.
Cover and press on a piece of wax paper over the tomato casserole. Leave wax paper on while cooking.
Cook in the microwave on high for 4-8 minutes or until hot and bubbly.
Let set for 3-5 minutes before serving.