Coarsely chop onions; peel and mince garlic. Place in a 3 quart microwave casserole. Cover with waxed paper. Microwave at High (100 percent power) 3 to 5 minutes until onions begin to look translucent. Scrub potatoes. Leaving peel on, cut potatoes into large dices. Add potatoes to onion, recover with waxed paper. Microwave on High 14 to 18 minutes until largest pieces are tender when pierced with fork. Set aside. Place sausage in a 1 1/2 quart microwave measuring bowl or casserole. Cover with waxed paper and microwave on High 5 to 8 minutes, breaking up with fork every 2 minutes until no pink remains in sausage. When cooked, crumble sausage loosely with fork, but do not drain (there should be about 1/4 cup drippings). To undrained sausage, add flour and salt. Mix well, then stir in milk. Microwave at High 8 to 12 minutes, stirring every 3 minutes until smooth and thickened. Pour sausage gravy over potatoes and onions and gently stir to mix well. Just before serving, return casserole to microwave to reheat at High 3 to 7 minutes, if necessary.