In a large skillet over medium high heat, cook the ground beef until no longer pink. Use a wooden spoon to break it apart. This takes about 5 minutes.

Add the diced onion and minced garlic to the skillet. Cook for 2 to 3 minutes until the onion is soft and translucent. Stir frequently to prevent burning.

Reduce the heat to medium. Add the butter and let it melt. Sprinkle the flour over the mixture and stir constantly for 1 minute.

Slowly pour in the milk while whisking. Keep whisking to avoid lumps. Add salt, pepper, paprika, and cayenne if using. Continue stirring until the sauce thickens, about 2 to 3 minutes.

Remove the skillet from the heat and stir in the shredded cheddar cheese until it melts completely.

Transfer the beef mixture to a 6 quart slow cooker. Add the frozen mixed vegetables, drained diced tomatoes, Worcestershire sauce, and uncooked elbow macaroni. Stir everything together until well combined.

Cover and cook on low for 3 to 4 hours, or on high for 1.5 to 2 hours. Check at the lower end of the time range to avoid overcooking the pasta.

If you want a crispy top, sprinkle breadcrumbs over the casserole and switch the slow cooker to high, uncovered, for 15 minutes. Or transfer to a baking dish and broil for 2 to 3 minutes. Watch closely.

Let the casserole sit for 5 minutes before serving. This allows the sauce to set slightly. Spoon into bowls and top with extra cheese if desired.