In a large bowl, combine ground beef, dry onion soup mix, breadcrumbs, eggs, milk, Worcestershire sauce, garlic powder, pepper, and shredded Swiss. Mix just until everything is combined — don't overwork the meat.
Form the mixture into a loaf shape and place it into your slow cooker (spray the insert first for easier cleanup). You can also use a slow cooker liner.
Cover and cook on low for 6–7 hours or on high for 3–4 hours, until the internal temperature reaches 160°F.
About 10–15 minutes before serving, lay the Swiss cheese slices over the top of the meatloaf, cover again, and let them melt.
Once the cheese is melted and gooey, turn off the slow cooker. Let the meatloaf rest for 5–10 minutes, then slice and serve.
