Brown ground beef in a large skillet over medium-high heat, breaking into crumbles as it cooks.
Add chopped onion to the skillet and cook 5 to 7 minutes until softened and translucent.
Stir in minced garlic during the last minute until fragrant, then drain excess grease.
Transfer beef and onion mixture to slow cooker; add frozen diced potatoes, drained corn, and undrained Rotel tomatoes.
Whisk together cream of mushroom soup and beef broth or milk until smooth; pour over ingredients in slow cooker.
Sprinkle paprika, parsley flakes (if using), chili powder, garlic powder, black pepper, and salt evenly over all ingredients.
Gently stir to combine all ingredients without leaving dry spots.
Cover and cook on LOW for 6 to 8 hours or on HIGH for 3 to 4 hours until potatoes are tender.
During the last 30 minutes of cooking, sprinkle 1 1/2 cups shredded cheddar cheese evenly over casserole and replace the lid.
Once cheese is melted and bubbly, top with remaining cheddar cheese and optional fried onions or bacon just before serving.
Serve warm as a main dish or hearty side, pairing well with a simple green salad or crusty bread.