In a large pan or skillet over medium-high heat, brown the lean ground beef, breaking it up as it cooks. Season generously with salt, pepper, and garlic powder.
Drain the fat from the cooked ground beef, then use a paper towel to wipe out the skillet. Melt the butter in the skillet, then whisk in the flour. Cook the roux for 2-3 minutes, or until golden brown, then whisk in the whole milk. Season with salt and pepper. Remove from heat and refrigerate until ready to use.
Place the cooked ground beef, along with the onion, carrot, celery, and potatoes, in the slow cooker. Cover with chicken broth and season with basil and parsley. Cover the slow cooker and cook on high for 3-4 hours, or until the potatoes are tender.
Stir in the refrigerated roux mixture, then add the cubed Velveeta cheese. Cover the slow cooker again and cook for another 30 minutes, or until the cheese is melted and the soup is thick and smooth.
Serve the soup hot, topped with grated cheddar cheese and bacon bits for added flavor and visual appeal. Enjoy the cheesy deliciousness!