Creamy brussel sprouts

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Ingredients:

2 cups brussel sprouts (one 10-ounce package frozen brussel sprouts)
1/4 cup chopped onions
1 tbsp margarine or butter
1/2 cup milk
1/4 cup dairy sour cream or plain yogurt
2 tsp all purpose flour
1/2 tsp horse radish mustard
1/8 tsp salt