Cook aromatics: Heat the olive oil in a large skillet over medium-high heat. Add the diced onion and cook for 3 minutes until softened.
Brown meat: Add the ground turkey. Break it apart with a spoon and cook for 6 minutes until no pink remains. Stir in the minced garlic and cook for 1 minute.
Cook vegetables: Add the sliced zucchini, Italian seasoning, salt, black pepper, and red pepper flakes. Toss to combine and cook for 4 minutes until the squash slightly softens.
Simmer sauce: Pour the undrained diced tomatoes into the pan. Reduce heat to medium-low and simmer uncovered for 5 minutes.
Garnish and serve: Remove from heat, sprinkle the grated Parmesan cheese evenly over the top, and serve immediately.
