Cook pasta in well-salted water until al dente. Reserve ½ cup pasta water, then drain.
Season chicken with salt and pepper. Heat olive oil in a large skillet over medium heat.
Sear chicken on both sides until golden and cooked through. Remove and slice.
In the same skillet, sauté onion until soft. Add garlic and cook until fragrant.
Pour in heavy cream. Stir in sun-dried tomatoes, Italian seasoning, and red pepper flakes. Simmer gently.
Add Parmesan and stir until smooth and creamy.
Return chicken and pasta to the skillet. Toss to coat, adding pasta water if needed.
Taste, adjust seasoning, and serve immediately.