Place bacon in a 1-quart casserole; cover with paper toweling.
Micro-cook til! crisp, about 1 1/2 minutes.
Remove bacon; drain.
Reserve drippings in casserole.
Crumble bacon; set aside.
In reserved drippings cook onion till tender, about 1 1/2 minutes.
Stir in vinegar and sugar; stir in spinach.
Micro-cook, covered, till heated through, 2 to 2 1/2 minutes.
Sprinkle with crumbled bacon.