PASTA: 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, but reduce the lowest indicated cooking time by 1 minute. Drain the pasta and set aside.
MUSHROOMS and TOMATOES: Heat a large pan over medium heat. Add 1 tbsp of oil from the sundried tomatoes. Once the 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 for another 1-2 minutes, or until the mushrooms are tender. Remove the contents from the pan and set aside on a plate.
CREAM SAUCE: Using the same pan, increase the 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 one minute to prevent lumps from forming.
CREAM SAUCE, CONT.: 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 the heat to medium-low. Let it simmer until the sauce thickens, stirring occasionally. Once thickened, reduce the heat to low and stir in the Italian seasoning and 5 tablespoons of Parmesan cheese. Season with salt and pepper to taste (I add ½ tsp salt and ¼ tsp pepper).
FINISH: Once the 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 ½ cup Parmesan cheese over everything.
ENJOY: Give everything a final stir and serve immediately, garnished with fresh thyme if desired.
Notes
Note 1: Parmesan cheese: Be sure to grab a block of Parmesan and grate it freshly. Use the small holes of a grater or a microplane to grate. Gently pack in a measuring cup for an accurate measurement.