Bring a large pot of salted water to a boil (use 1 tbsp salt for 12 cups of water). Add the penne pasta and cook according to package instructions, minus 1 minute. Drain the pasta and set aside.
Heat a large pan over medium heat. Add 1 tbsp oil from the sundried tomatoes. Once oil is hot, add the sliced mushrooms. Cook for 3 minutes, stirring occasionally. Add the chopped sundried tomatoes to the pan and continue to cook another 1–2 minutes, or until the mushrooms are tender. Remove the contents from the pan and set aside on a plate.
Using the same pan, increase heat to medium-high. Add another tbsp of sundried tomato oil and the unsalted butter. When the butter is melted, add the minced garlic and sauté until fragrant (about 30 seconds). Sprinkle the flour over the mixture. Whisk constantly for 1 minute to prevent lumps from forming.
Gradually pour in the chicken broth, continuing to whisk, followed by the milk. Keep whisking to combine. Bring the mixture to a boil, then immediately reduce heat to medium-low. Let it simmer until the sauce thickens, stirring occasionally. Once thickened, reduce heat to low and stir in the Italian seasoning and 5 tablespoons of Parmesan cheese. Season with salt and pepper to taste (I add 1/2 tsp salt and 1/4 tsp pepper).
Once cheese is melted and sauce is thickened, return the cooked pasta, mushrooms, chicken, and spinach to the pan. Toss to coat in sauce and allow spinach to wilt, about 1–3 minutes over medium heat. Stir the ingredients until combined. Remove pan from heat and sprinkle 1/2 cup Parmesan cheese over everything.
Give everything a final stir and serve immediately, garnished with fresh thyme if desired.
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Notes
Note 1: Be sure to grab a block of Parmesan and grate it freshly. Use the small holes of a grater or a zester to grate. Gently pack in a measuring cup for an accurate measurement.Storage: Store leftovers in an airtight container in the fridge for up to 4 days. Reheat gently on the stove or in the microwave, adding a splash of cream or broth to refresh the sauce.