Spray the inside of a six-quart slow cooker with nonstick cooking spray.
Heat olive oil in a large skillet over medium-high heat. Add the ground beef and cook for six to eight minutes, breaking it into small pieces, until fully cooked. Drain excess grease.
Add the diced onion and cook for three to four minutes. Stir in the garlic, paprika, garlic powder, thyme, salt, and pepper. Cook for another minute.
In a mixing bowl, whisk together the cream of mushroom soup, cheddar cheese soup, milk, beef broth, and Worcestershire sauce until smooth.
Spread ¼ cup of sauce over the bottom of the slow cooker. Add half of the sliced potatoes, followed by half of the ground beef mixture.
Spread half of the remaining sauce over the beef and sprinkle with ½ cup of shredded cheddar cheese.
Repeat with the remaining potatoes, ground beef, and sauce. Sprinkle with another ½ cup of cheddar cheese.
Cover and cook on LOW for five to six hours or HIGH for three to four hours, until the potatoes are fork-tender.
Sprinkle the remaining 1½ cups of cheddar cheese over the casserole. Cover and cook for another 15 to 20 minutes, until melted.
Turn off the slow cooker and allow the casserole to rest for ten minutes. Garnish with parsley, if desired, and serve warm.