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In small skillet, cook onion in 1 tablespoon butter until soft.
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In large bowl, combine onion with next 10 ingredients. (Mixture is very soft). Shape into 1 1/2-inch balls; arrange on jelly-roll pan lightly sprayed with vegetable cooking spray. Bake in a preheated 400 F oven until cooked through and lightly browned (14-16 minutes).
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Meanwhile, melt 3 tablespoons butter in large skillet; stir in flour. Gradually whisk in beef broth. Cook over medium heat, stirring constantly, until mixture thickens. Stir in whipping cream and 1/2 teaspoon dill weed; simmer 5 minutes. (Have gravy finished when meatballs come out of oven.)
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Scrape meatballs and browned bits into gravy; stir to combine. Spoon into 3-quart casserole. Refrigerate, covered, several hours or overnight.
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Reheat in a preheated 325 F oven until heated through (40-50 minutes).