Step 1: Prepare Meatballs (if homemade): In a large bowl, combine ground meat, breadcrumbs, 1/4 cup milk, egg, half of the chopped onion, 1 minced garlic clove, salt, and pepper. Mix until just combined, then form into 1-inch meatballs.
Step 2: Cook Meatballs: Heat olive oil in a large skillet or Dutch oven over medium-high heat. Brown the meatballs on all sides. Remove and set aside.
Step 3: Make Sauce: Add remaining onion and garlic to the skillet and sauté until softened, about 3-4 minutes. Stir in tomato sauce, beef broth, Italian seasoning, salt, and pepper. Bring to a simmer.
Step 4: Simmer Meatballs: Return the browned meatballs to the sauce. Reduce heat to low, cover, and simmer for 15-20 minutes, or until meatballs are cooked through.
Step 5: Prepare Mashed Potatoes: While meatballs simmer, boil quartered potatoes in salted water until very tender, about 15-20 minutes. Drain well.
Step 6: Mash Potatoes: Return drained potatoes to the pot. Add 1/2 cup milk, butter, sour cream (if using), salt, and pepper. Mash until smooth and creamy. Stir in 1/2 cup shredded cheddar cheese until melted.
Step 7: Assemble Bake: Preheat oven to 375°F (190°C). Pour the saucy meatballs into a 9x13 inch baking dish. Carefully spread the mashed potatoes evenly over the meatballs.
Step 8: Bake: Sprinkle the remaining 1/2 cup cheddar cheese (and mozzarella, if using) over the mashed potatoes. Bake for 20-25 minutes, or until the topping is golden brown and bubbly. If desired, broil for the last 2-3 minutes for an extra crispy topping.
Step 9: Serve: Let the casserole rest for 5-10 minutes before serving. Enjoy hot!