In a large bowl, combine the ground beef, breadcrumbs, French onion soup mix, eggs, milk, Worcestershire sauce, garlic powder, onion powder, salt, and pepper. Mix until just combined; avoid overmixing to keep the meatloaf tender.
Form the mixture into a loaf shape that will fit into your slow cooker.
In a skillet over medium heat, add olive oil and sauté the sliced onions until they become golden brown and caramelized, about 10–15 minutes. Stir occasionally to prevent burning.
Place the meatloaf into the slow cooker. Top with the caramelized onions.
Cover and cook on low for 6–7 hours or on high for 3–4 hours, until the meatloaf reaches an internal temperature of 160°F (71°C).
About 10 minutes before serving, place the Swiss cheese slices on top of the meatloaf. Cover and allow the cheese to melt.
Carefully remove the meatloaf from the slow cooker. Slice and serve hot.