Mix the ground beef, onion, green bell pepper, breadcrumbs, 1/2 cup milk, garlic, 1/4 cup ketchup, Worcestershire sauce, salt, and black pepper in a large bowl until evenly combined.
Shape the mixture into a loaf and place it in the slow cooker. Spread the remaining 1/4 cup ketchup over the top.
Cover and cook on low for 6 hours until the internal temperature reaches 160 degrees.
While the meatloaf cooks, place the sweet potatoes in a pot of water and boil until soft, about 15 minutes. Drain, then mash with 1 tablespoon butter and 1/4 cup milk until smooth.
Steam the green beans in a separate pot or steamer basket for 8 minutes until tender. Toss with the remaining 1 tablespoon butter.
Slice the meatloaf and serve with the mashed sweet potatoes and green beans on the side.