Step 1: Heat olive oil in a large skillet or Dutch oven over medium-high heat. Add the ground turkey, breaking it up with a spoon. Cook until browned and no longer pink, about 5-7 minutes. Drain any excess fat if necessary.
Step 2: Add the chopped onion and bell pepper to the skillet. Sauté for 5-7 minutes, until the vegetables soften and become fragrant. Stir in the minced garlic and cook for another minute until aromatic.
Step 3: Stir in the crushed tomatoes, Italian seasoning, red pepper flakes (if using), salt, and black pepper. Bring the mixture to a simmer, then reduce the heat to low, cover, and let it cook for at least 15-20 minutes, allowing the flavors to meld beautifully.
Step 4: If using fresh spinach, stir it into the sauce during the last 5 minutes of cooking, until wilted.
Step 5: Uncover the skillet. Tear or slice the fresh mozzarella into pieces and dot them over the top of the turkey mixture. Cover again for 2-3 minutes, or until the mozzarella is melted and bubbly. For extra browning, you can briefly place the skillet under the broiler (watch carefully!).
Step 6: Garnish with fresh basil or parsley, if desired. Serve hot.