Finely chop onion, mince garlic, and slightly whisk egg.
Add beef, bread crumbs, tomato sauce, onion, garlic, egg, salt, and pepper to a large bowl; mix well (you may want to use your hands).
Spray a slow cooker with cooking spray and then add the beef mixture. Shape into a loaf, leaving some space around the edges of the slow cooker. Cover and cook on low for 6-7 hours or until cooked through and no longer pink.
A bit before the meatloaf is finished cooking, peel and chop the potatoes. Add to a saucepan and cover with water. Boil until fork tender, then drain; add the butter and salt, and mash together.
In a large bowl, toss together the spinach, balsamic vinegar, and olive oil.
Serve meatloaf with spinach salad, sweet potatoes, and orange wedges on the side.