1. Put butter and onion into a 1 1/2-quart glass casserole. Cook uncovered in microwave oven 2 minutes at HIGH; stir after 1 minute.
2. Blend flour into onion butter and stir in pineapple juice. Cook uncovered in microwave oven 1 minute at HIGH, or until thick.
3. Stir pineapple tidbits, brown sugar and ham into sauce. Pile in center of an 8-inch round glass cake dish. Put sliced yams around outer edge. Cover with waxed paper. Heat in microwave oven to serving temperature (6 to 8 minutes at BAKE).