1. Combine the ground beef with the chopped onion, egg, bread crumbs, salt and pepper. Shape into meatballs. Brown in butter. Drain well.
2. Stir the tomatoes, 1 cup water, beef base, and seasonings together. Place the carrots, potatoes and sliced onion in the bottom of the slow cooker. Top with the meatballs.
3. Pour the tomato mixture over; cover. Cook on LOW for 8 to 10 hours. Remove the meatballs (use a slotted spoon).
4. Make a smooth paste of the cornstarch and 1/4 cup water. Stir into the vegetables; cover. Cook on HIGH until thickened (about 10 minutes). Return the meatballs. Stir.