You can use Vidalia onions if you can't find Maui onions. Combine onion slices and butter in a 3 quart casserole. Microwave on high 10-12 minutes, until onion is soft. Stir, add broth and seasonings. Cover with lid or a tight seal of plastic wrap. Microwave on high 12 minutes or so, until soup boils. Fill each serving bowl. Top with slice of bread and slice of mozzarella cheese on top. Microwave 2-4 bowls at a time on high 30-45 seconds to melt cheese. Serve.