Start by preheating your oven to 350F and lining a baking tray with parchment paper.
Remove the stems from the mushrooms and reserve the stems for later. Place the mushroom caps top side down on the prepared baking sheet.
Chop up the mushroom stems very finely and add them to a large bowl with the ground beef, garlic, egg, panko crumbs, ¼ cup marinara sauce and the spices.
Mix the meatball mixture until smooth. Shape 24 meatballs, about 1-inch in diameter. Place them in the mushroom cavities. Press down gently so that they are well secured.
Divide the remaining ¾ cup of marinara sauce evenly over the top of the meatballs. Top each meatball with grated mozzarella, dividing it as equally as possible.
Place the prepared Meatball Stuffed Mushrooms into the preheated oven. Bake for 16-18 minutes, or until the internal temperature of the meatball reaches a minimum of 160F.
Remove from the oven and sprinkled the meatballs with finely chopped fresh basil.