In large saucepan or Dutch oven pour enough water over ribs to cover.
Bring to boiling; reduce heat.
Cover; simmer for 45 minutes.
In small saucepan combine sugar, vinegar, prepared mustard, chopped onion, minced garlic, celery seed, salt, and ground turmeric.
Bring mixture to boiling, stirring till sugar dissolves.
Place ribs, meaty side up, in shallow roasting pan.
Brush some of the mustard mixture over ribs.
Roast, uncovered, at 350° about 15 minutes.
Top with thinly sliced onion and some additional mustard mixture.
Roast 15 minutes more, brushing occasionally with mustard mixture.
Transfer ribs to warm platter; cut ribs into serving-size pieces.
Reheat remaining mustard mixture and pass with ribs.