Prep the base: Add frozen meatballs to the slow cooker. Scatter diced onion and sliced mushrooms over the top. Sprinkle in garlic (or garlic powder).

Mix the sauce: In a bowl, whisk beef broth, Worcestershire, Dijon, paprika, onion powder, a pinch of salt, and pepper. Pour over the meatballs and veggies.

Cook low and slow: Cover and cook on Low for 5–6 hours or High for 2.5–3 hours, until the onions are soft and the meatballs are hot through.

Thicken the sauce: Melt butter in a small saucepan. Whisk in flour and cook 1 minute. Ladle in 1 cup of hot cooking liquid from the slow cooker, whisking until smooth and slightly thick. Stir the mixture back into the crockpot. (For cornstarch: whisk 1 tablespoon cornstarch with 2 tablespoons cold water, then stir into the crockpot and cook 10–15 minutes.)

Add the creaminess: Turn the slow cooker to Warm. Stir in sour cream until fully blended and the sauce is velvety. Taste and adjust salt, pepper, or Worcestershire.

Cook the noodles: Boil egg noodles according to package directions just before serving. Drain and toss with a little butter and salt for extra flavor.

Serve and garnish: Spoon meatballs and sauce over noodles, rice, or potatoes. Finish with chopped parsley and a crack of black pepper.