Bring a large pot of salted water to a boil. Cook the penne according to package directions minus 1 minute so it is just shy of al dente. Drain and set aside.
Heat a large pan over medium heat and add 1 tablespoon of the oil from the sun-dried tomato jar. When hot, add the sliced mushrooms and cook, stirring occasionally, about 3 minutes until they begin to soften.
Add the chopped sun-dried tomatoes to the mushrooms and cook 1–2 more minutes until mushrooms are tender. Transfer the mushrooms and tomatoes to a plate and set aside.
Return the pan to medium-high heat and add another tablespoon of sun-dried tomato oil and the butter. When the butter melts, add the minced garlic and sauté about 30 seconds until fragrant.
Sprinkle the flour over the butter and garlic and whisk continuously for about 1 minute to form a roux and prevent lumps.
Slowly whisk in the chicken stock, then the milk, stirring constantly until smooth. Bring to a boil, then reduce heat to medium-low and simmer until the sauce thickens, stirring occasionally.
Lower the heat and stir in the Italian seasoning and 5 tablespoons grated Parmesan until melted. Season the sauce with salt and pepper to taste (about 1/2 tsp salt and 1/4 tsp pepper suggested).
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 1/2 cup freshly grated Parmesan over the pasta, give a final stir, and serve immediately garnished with fresh thyme if desired.