Boil the tortellini: Bring a large pot of salted water to a boil.
Cook the tortellini according to package directions until just al dente. Reserve 1/2 cup pasta water, then drain and set aside.
Start the aromatics: In a large skillet or Dutch oven, heat olive oil over medium. Add the diced onion with a pinch of salt and cook 3–4 minutes until softened.
Add garlic and cook 30–60 seconds until fragrant.
Brown the turkey: Add the ground turkey to the pan. Season with 1 teaspoon salt, 1/2 teaspoon black pepper, Italian seasoning, and red pepper flakes if using. Cook, breaking it up with a spoon, until browned and cooked through, about 5–7 minutes.
Build the sauce: Stir in tomato paste and cook 1 minute to caramelize.
Pour in crushed tomatoes and 1/2 cup chicken broth. Simmer 5–8 minutes, stirring occasionally, until slightly thickened. Adjust consistency with more broth or reserved pasta water if needed.
Add greens and tortellini: Stir in spinach and let it wilt, 1–2 minutes.
Gently fold in the cooked tortellini until coated in sauce. Taste and adjust seasoning with more salt and pepper.
Finish and serve: Off the heat, add a squeeze of lemon if you like brightness. Top with grated Parmesan and torn basil or parsley.
Serve hot with extra cheese at the table.