Heat oven to 375 degrees F. Heat 1 tablespoon oil in a deep 12-inch oven-proof skillet over medium-low heat. Add the onion, mushrooms, and garlic and cook, stirring occasionally, until the mushrooms are tender and their liquid is nearly evaporated, 4 to 5 minutes. Season with 1/4 teaspoon salt and pepper to taste and set aside to cool slightly.
Meanwhile, in a large bowl, whisk together the egg, mustard, 1/4 teaspoon each salt and pepper. Stir in the bread crumbs, fresh parsley, basil and fennel seed (crush it with your fingers as you add it). Add the ground turkey, sprinkle with the Parmesan or Pecorino Romano cheese, and mix to combine. Fold in the mushroom mixture. Form the mixture into 16 meatballs and set them aside on a large piece of foil.
In the same skillet you used to cook the mushroom mixture, heat the remaining olive oil over medium-high heat. Add the meatballs and brown on all sides. There is no need to cook them through at this point, just cook for 3 to 4 minutes, turning with tongs until nicely browned on all sides.
Pour the sauce into the skillet over the meatballs. For very saucy meatballs add the entire jar of marinara or add less to your liking. Top with mozzarella cheese, transfer the uncovered skillet to the oven and bake for 15 to 20 minutes, or until meatballs are cooked through and cheese has melted.
Garnish with fresh basil and serve.