In a large skillet over medium-high heat, brown the ground beef, breaking it into crumbles as it cooks for 5-7 minutes total. Add the chopped yellow onion and cook for another 5 minutes until softened and translucent. Stir in the minced garlic during the last minute until fragrant. Carefully drain off all excess grease from the skillet.

Transfer the cooked and drained ground beef and onion mixture to a 6-quart or larger slow cooker. Add the frozen diced potatoes (no need to thaw), drained corn, and entire undrained can of Rotel directly into the slow cooker.

In a medium bowl, whisk together the condensed cream of mushroom soup and beef broth (or milk) until smooth and lump-free. Pour this mixture evenly over the ingredients in the slow cooker.

Sprinkle the paprika, parsley flakes (if using), chili powder, garlic powder, black pepper, and 1/2 teaspoon salt over all ingredients in the slow cooker. Gently stir all ingredients together until well combined and potatoes are evenly coated with the sauce.

Cover with lid and cook on LOW for 6-8 hours or HIGH for 3-4 hours. The casserole is done when potatoes are fork-tender, easily pierced throughout, and the mixture is hot and bubbling around the edges.

During the last 30 minutes of cooking, sprinkle 1 1/2 cups of shredded cheddar cheese evenly over the top. Replace lid and continue cooking until cheese is completely melted and bubbly. If desired, top with remaining cheese, crispy fried onions, or crumbled bacon just before serving.