Bring a large pot of salted water to a rolling boil. Add penne and cook for 8-10 minutes until just shy of al dente. Reserve 1 cup of pasta water, then drain.

Heat olive oil in a large skillet over medium heat. Add diced onion and cook for 5 minutes, stirring occasionally, until soft and translucent.

Increase heat to medium-high and add ground turkey to the skillet. Break up with a wooden spoon and cook for 6-8 minutes until browned with no pink remaining. Season with salt, pepper, and Italian seasoning. Add minced garlic and cook for 1 minute until fragrant.

Sprinkle flour evenly over the turkey and stir to coat. Cook for 1 minute. Gradually pour in chicken broth, stirring constantly to prevent lumps. Add tomato sauce and bring to a gentle simmer. Cook for 3-5 minutes until sauce thickens.

Add drained penne to the skillet and toss to coat with sauce. Pour in half-and-half and stir gently. Add reserved pasta water a few tablespoons at a time if needed to reach desired consistency.

Reduce heat to low. Stir in half of the mozzarella and cheddar, folding gently until melted. Sprinkle remaining cheese over the top, cover the skillet, and let sit for 2 minutes off the heat.

Remove lid and garnish with chopped fresh parsley and red chili flakes if using. Serve immediately while hot and cheese is still gooey.