Heat the oven up to 400 degrees Fahrenheit and get a 9x13 casserole dish ready.
Sauté onion in oil in a deep skillet with a heavy bottom or a dutch oven for about 2 minutes, then add chopped cabbage and keep cooking until softened, about 5-7 more minutes.
Add diced canned tomatoes, tomato sauce and broth, and cook together on medium heat for 5 minutes.
Make the meatballs. In a large bowl mix together raw ground meat, cooked brown rice, the egg, marjoram, garlic, paprika, salt and pepper and mix with your fingers until combined. Don’t overmix!
Using a medium cookie scoop, drop the meat and rice mixture balls onto a tray or a plate and roll into meatballs the size of a golf ball with wet fingers.
Assemble the casserole. Pour half of the cabbage tomato sauce in a 9x13 casserole dish. Place the raw meatballs in the sauce in one layer, top with remaining sauce and cover with tin foil.
Bake covered in preheated oven for 40 minutes or until bubbly. Uncover, bake 10-15 more minutes and serve hot!