In a skillet, heat olive oil over medium high heat. Add ground beef and diced onion and cook, breaking up the beef, until no pink remains and the onions are translucent. Add garlic and cook another 30 seconds, just until fragrant.

Transfer to the slow cooker and season with salt and pepper. Stir in Velveeta, ketchup, and mustard.

Cover and cook on low for 3 - 4 hours, stirring about once an hour. Serve on buns with more cheese and pickles!