In a skillet over medium heat, melt butter and add thinly sliced onions. Cook, stirring occasionally, until onions are soft and deep golden brown — about 20–25 minutes. Add garlic in the last minute.

In a large bowl, mix ground beef, breadcrumbs, milk, eggs, Dijon mustard, French onion soup mix (or Worcestershire + bouillon), salt, pepper, and thyme. Stir in about half the caramelized onions so you get pockets of onion-y richness in the loaf. Don’t overmix — use a fork or your hands gently until just combined.

Shape the mixture into a loaf that will comfortably fit your slow cooker base. Place it directly in the Crockpot. Pour beef broth around (not over) the loaf.

Cover and cook on LOW for 3–4 hours, or HIGH for 2–3 hours, until the internal temperature reaches 160°F (71°C).

In a small bowl, mix brown sugar and ketchup. Gently brush the glaze over the top of the meatloaf, then add the remaining caramelized onions. Lay Swiss cheese slices over the top, cover, and let melt for 5–10 minutes.

Carefully remove the meatloaf to a cutting board and let rest 10 minutes to hold its shape. Slice, drizzle with some of the cooking juices, and serve.