Bring a large pot of salted water to a boil (about 1 tbsp salt per 12 cups water). Add the penne and cook according to package directions minus 1 minute. Drain and set aside.
Heat a large skillet over medium heat and add 1 tablespoon of oil reserved from the sun-dried tomato jar. When hot, add the sliced mushrooms and cook, stirring occasionally, about 3 minutes.
Add the coarsely chopped sun-dried tomatoes to the mushrooms and cook 1–2 minutes more, until mushrooms are tender. Transfer the mixture to a plate and set aside.
Return the skillet to medium-high heat and add another tablespoon of the sun-dried tomato oil and the butter. When the butter melts, stir in the minced garlic and cook about 30 seconds until fragrant.
Sprinkle the flour over the butter-garlic mixture and whisk constantly for about 1 minute to form a roux and prevent lumps.
Gradually whisk in the chicken stock, then the milk, whisking constantly until smooth. Bring to a brief boil, then reduce heat to medium-low and simmer until the sauce thickens.
Reduce heat to low and stir in the Italian seasoning and 5 tablespoons grated Parmesan. Season with salt and pepper to taste (about 1/2 tsp salt and 1/4 tsp pepper suggested). Stir until cheese melts and sauce is smooth.
Add the cooked pasta, reserved mushrooms and sun-dried tomatoes, diced chicken, and packed spinach to the sauce. Toss over medium heat until everything is coated and the spinach wilts, about 1–3 minutes.
Remove from heat, sprinkle the remaining 1/2 cup grated Parmesan over the pasta, give a final stir, and serve immediately garnished with fresh thyme if desired.
