Heat 1 tablespoon of olive oil in a large, deep skillet or Dutch oven over medium-high heat. Add the sliced mushrooms in a single layer and let them cook undisturbed for 3-4 minutes until golden brown on the bottom. Stir and cook for another 2-3 minutes until tender and browned all over. Season with a pinch of salt and transfer to a plate.

In the same pan, add the remaining 1 tablespoon of olive oil and the butter over medium heat. Once the butter melts, add the minced garlic and cook for 30-60 seconds until fragrant, stirring constantly to prevent burning.

Pour in the vegetable broth and use a wooden spoon to scrape up any browned bits from the bottom of the pan. Add the penne pasta, thyme, salt, pepper, and red pepper flakes (if using). Stir to combine and bring to a boil.

Reduce heat to medium, cover the pan, and let the pasta cook for 12-14 minutes, stirring every 3-4 minutes to prevent sticking. The pasta is done when it's al dente and most of the liquid has been absorbed.

Reduce heat to low. Pour in the heavy cream and sprinkle in the grated Parmesan cheese. Stir until the cheese melts and the sauce becomes creamy and glossy, about 1-2 minutes.

Return the sautéed mushrooms to the pan. Add the fresh spinach in handfuls, stirring after each addition until wilted, about 1-2 minutes total. Taste and adjust salt and pepper as needed.

Divide the pasta among bowls and top with extra Parmesan cheese and a sprinkle of fresh thyme. Serve immediately while hot and creamy.