Melt butter in a skillet over medium heat. Add sliced onions and cook for 15–20 minutes, stirring occasionally, until golden brown and reduced in volume. Lower heat if they brown too quickly. Set aside to cool.
In a large bowl, combine ground beef, breadcrumbs, salt, pepper, garlic powder, and onion powder. Mix gently by hand or with a fork just until combined. Don’t overmix.
Grease the bottom of your slow cooker. Layer half the caramelized onions, then place the shaped meatloaf on top. Cover with remaining onions.
Sprinkle shredded Swiss cheese evenly over the top, covering the onions completely.
Cover and cook on low for 6–8 hours or high for 3–4 hours, until the meatloaf reaches 165°F and the cheese is bubbly.
Let rest for 5–10 minutes before slicing. Garnish with parsley or more cheese. Serve with mashed potatoes, roasted veggies, or crusty bread.