Bring a large pot of salted water to a boil and cook the penne according to package instructions until al dente; drain and set aside.
In a bowl, toss the cubed chicken with 1/2 teaspoon salt and 1/2 teaspoon ground black pepper until evenly coated.
Heat 1 tablespoon oil in a wide cast iron pan over medium-high heat. Add the seasoned chicken and cook, stirring occasionally, until browned and cooked through, about 5–7 minutes; transfer to a plate and keep warm.
Reduce heat to medium and add 1 tablespoon butter to the same pan. When melted, add the chopped garlic and onion and sauté until fragrant and translucent, about 2 minutes.
Add the chopped sun-dried tomatoes, then pour in the heavy cream and bring to a gentle simmer for about 2 minutes.
Stir in the shredded Parmesan until melted, then add the baby spinach and cook until wilted, about 1–2 minutes.
Season the sauce with 1/2 teaspoon ground black pepper, 1/2 teaspoon red chili flakes, 1/2 teaspoon Italian seasoning, and additional salt to taste; simmer the sauce 3–5 minutes to thicken slightly.
Add the cooked penne and the cooked chicken to the sauce, toss to combine, and cook for 1–2 minutes to heat through.
Transfer to a serving bowl and garnish with fresh basil and extra Parmesan if desired.