In saucepan, bring the water and 1/2 tsp. salt to boiling. Add cabbage and onion; cook, covered, till crisp-tender, 7 to 8 minutes. Drain, reserving cooking liquid. Add enough water to cooking liquid to make 3/4-cup. In same saucepan blend together sour cream, flour, mustard, caraway seed, 1/2 tsp. salt, and pepper. Stir in cooking liquid. Fold in cabbage and onion.
Turn mixture into a 1 1/2-quart casserole. Bake covered at 350 till heated through, about 20 minutes, stirring once. Sprinkle generously with paprika.