Dice the yellow onion finely and mince the 3 garlic cloves. Heat 1 tablespoon of olive oil in a skillet over medium heat and sauté the onion for about 5 minutes until softened and translucent. Add the minced garlic and cook for 1 minute more, then transfer to a plate to cool completely.

In a large mixing bowl, combine the egg, heavy cream, panko breadcrumbs, grated Parmesan cheese, fresh parsley, Italian seasoning, and mustard powder. Stir until well combined. Add ¾ cup of the ricotta and the cooled onion-garlic mixture from Step 1, folding gently to incorporate.

Using a cookie scoop or your hands, roll the mixture into 1½-inch balls and place them on a parchment-lined baking sheet or plate. Once all meatballs are shaped, refrigerate them for 15 minutes.

Heat 8 tablespoons of olive oil in a large skillet over medium-high heat until shimmering. Working in batches to avoid overcrowding, carefully add the chilled meatballs and sear for about 1½ minutes per side until they develop a golden-brown crust.

Pour off most of the oil from the skillet, leaving just a light coating. Add the marinara sauce and ¼ cup water, stirring to combine and scrape up any flavorful browned bits from the bottom of the pan. Carefully return the browned meatballs to the skillet, nestling them into the sauce. Spoon some sauce over the top of each meatball to coat them. Reduce heat to medium, cover the skillet partially with a lid, and simmer for 30 minutes, stirring gently halfway through.

After the meatballs have simmered for about 15 minutes, dollop the remaining 4 oz of ricotta evenly over the top of the meatballs. Let everything continue cooking on medium heat, partially covered, for the remaining 15 minutes until the meatballs are cooked through and the ricotta has warmed and created a luxurious sauce.