Preheat the oven to 450 degrees.
Mix the beef, sausage, onions, garlic, thyme, parsley, egg, Parmesan, breadcrumbs, ricotta and salt in a bowl together with your hands (like mixing play dough). Form 1 1/2-inch inch balls out of the meat mixture. Heat a large ovenproof skillet (preferably cast iron) with a bit of olive oil. Add the meatballs and brown on all sides, about 3 minutes. Place the skillet in the oven to finish cooking, 5 to 7 minutes.
Remove and toss with the red sauce. Place on the toasted slider buns and sprinkle with Parmesan and parsley.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.
Recipe courtesy of Domenica Catelli, owner of Catelli's, in Geyserville, CA