Melt the 2 tablespoons of butter in a skillet over medium heat. Add the thinly sliced medium onion and cook for 10-12 minutes, stirring occasionally, until golden and caramelized. Transfer the onions to a bowl and set aside.

In a large bowl, combine 2 pounds of ground beef, 1 packet (1 oz) dry onion soup mix, 2 minced garlic cloves, 1 cup breadcrumbs, 2 large eggs, 1/2 cup milk, 1/2 cup grated Parmesan cheese, 2 tablespoons ketchup, 1 tablespoon Worcestershire sauce, 1 teaspoon dried thyme or Italian seasoning, 1/2 teaspoon black pepper, and 1/2 teaspoon salt. If using, gently fold in the optional 1/2 cup shredded mozzarella cheese. Mix just until combined to keep the meatloaf tender.

Shape the meat mixture into a loaf that will fit comfortably in your crockpot and place it in the pot. Pour 1 cup of beef broth around (not over) the meatloaf and scatter the caramelized onions evenly on top.

Cover the crockpot and cook on low for 5-6 hours (or on high for 3-4 hours). When about 15 minutes remain, spread a little extra ketchup on top of the loaf and sprinkle with 1 cup of shredded Gruyère or Swiss cheese. Cover again until the cheese is melted and bubbly.

Remove the meatloaf from the crockpot and serve it with your favorite sides, such as steamed greens, mashed potatoes, or crusty French bread to soak up the delicious juices.