Make the sauce: In a large saucepan over medium heat, saute the garlic and red pepper flakes in the oil 1 minute, stirring. (Do not brown the garlic.) Stir in the tomatoes and salt. Bring the sauce to a boil, reduce the heat to low and simmer, stirring occasionally, 15 minutes.
Meanwhile, make the meatballs: Preheat the oven to 450 degrees F. In a large bowl, mix the beef, sausage, onion, garlic, thyme, parsley, egg, parmesan, breadcrumbs, ricotta and salt with your hands. Form 1 1/2-inch balls out of the meat mixture. Heat a large ovenproof skillet (preferably cast iron) with a bit of olive oil. Working in batches, add the meatballs and brown on all sides, about 4 minutes. Return all the meatballs to the skillet and place in the oven to finish cooking, 10 minutes.
Toss the meatballs with 3 cups of the prepared sauce. Place on the toasted slider buns and sprinkle with parmesan and parsley.