Gently mix beef, ricotta, breadcrumbs, egg, meatball Parmesan, 1/2 teaspoon oregano, meatball salt and pepper. Shape 24 equal meatballs and refrigerate while making the sauce.
Heat olive oil in a saucepan over medium heat and cook onion for 6 minutes. Stir in garlic for 30 seconds, add tomatoes, broth, remaining oregano and sauce salt and bring to a simmer.
Pour hot marinara into a 5-quart slow cooker set to HIGH. Lower the raw meatballs into the sauce in no more than 2 layers, cover and cook on HIGH for about 3 hours until the largest centers reach 160°F; avoid stirring for the first 2 hours, then turn gently once.
During the final 40 minutes, bring polenta water and polenta salt to a boil in a heavy saucepan. Whisk in dry polenta gradually, reduce to low and cook for 30 to 35 minutes, stirring frequently, until the grains are tender.
Fold butter and polenta Parmesan into the hot polenta. Check the marinara and meatballs are hot throughout and stir only enough to coat without breaking them.
Divide polenta among 6 shallow bowls and add 4 meatballs and marinara to each. Serve immediately while the polenta is spoonable.