Preheat your oven to 375°F (190°C).
In a large mixing bowl, combine the ground beef, Italian sausage, ricotta cheese, grated Parmesan, egg, grated onion, minced garlic, breadcrumbs, chopped parsley, dried oregano, salt, and pepper. Mix until all ingredients are well combined but do not overmix to keep the meatballs tender.
Using your hands or a cookie scoop, form the mixture into meatballs about 1½ inches in diameter and place them on a baking sheet lined with parchment paper.
Bake the meatballs in the preheated oven for 20-25 minutes, or until they are browned and cooked through. You can also air fryer them for 10-12 minutes, shaking the basket halfway through, until the meatballs are browned and cooked through.
While the meatballs are cooking, heat 2 tablespoons of olive oil in a large skillet or Dutch oven over medium heat. Add the finely chopped onion and sauté until softened, about 5 minutes. Add the minced garlic and cook for another 1-2 minutes until fragrant.
Stir in the crushed tomatoes, brown sugar, heavy cream, Italian seasoning, salt, and pepper. Bring the sauce to a simmer and let it cook for 10-15 minutes, allowing the flavors to meld together.
Once the meatballs are done cooking, gently place them into the simmering marinara sauce. Spoon some of the sauce over the meatballs to coat them.
Let the meatballs simmer in the sauce for an additional 10 minutes to absorb the flavors. If desired, you can add dollops of additional ricotta cheese on top of the meatballs and let them melt slightly.
Garnish the dish with fresh chopped parsley. Serve the ricotta meatballs hot, either on their own, over pasta, or with crusty bread on the side.