Bring a large pot of water to a boil. Add 1 teaspoon salt and cook the penne until one minute shy of al dente. Reserve 3/4 cup pasta water, then drain.

While the water heats, pat the chicken dry. Season it with the remaining salt, black pepper, and 1/2 teaspoon Italian seasoning.

Heat the olive oil in a 12-inch skillet over medium-high heat. Sear the chicken in one layer for 6 to 8 minutes, turning occasionally, until browned and 165°F inside. Transfer it to a plate.

Reduce the heat to medium. Add the garlic, sun-dried tomatoes, remaining Italian seasoning, and red pepper flakes. Cook for 1 minute.

Sprinkle in the flour and stir for 30 seconds, coating the tomatoes and cooking away the raw flour smell.

Gradually whisk in the chicken broth while scraping up the browned bits. Simmer for 3 to 4 minutes, until lightly thickened.

Lower the heat. Stir in the half-and-half, followed by the Parmesan, until the sauce is smooth. Do not let it boil.

Add the spinach, chicken, and drained penne. Toss over low heat for 1 to 2 minutes, adding reserved pasta water a few tablespoons at a time if the sauce is too tight.