Dreamy Mushroom Pasta

1. Bring a large pot of water to a boil, adding about 1 tbsp of salt to the water. Cook the pasta until al dente according to package directions. Make sure to reserve 1/4 cup of pasta water before draining.
2. In a large saute pan with high sides, melt 3 tbsp of butter. Add the chopped onion and saute for 2 minutes until they start to become fragrant and translucent in color.
3. Add the mushrooms to the pan, moving them gently around to coat with butter. If the pan seems dry, you can add another tablespoon of butter. Sprinkle about 1 tsp kosher salt over top and 1/4 tsp pepper over top, stir to distribute.
4. Let the mushrooms saute for about 3 minutes then add the wine to the pan. Let it come to a simmer, allowing the wine to cook down until it is absorbed into the mushrooms, about 10 minutes.
5. Then add in the spinach, stirring as the spinach wilts into the pan.
6. Add the heavy cream and dijon mustard to the pan, stirring to incorporate and bring to a gentle simmer for about 3-4 minutes; the sauce should thicken.
7. Add the cooked pasta to the pan and toss to coat. Then add in the mascarpone cheese and reserved pasta water. Toss to coat once more, letting the mascarpone melt into the pan.
8. Turn off the heat and add the parmesan cheese, continuing to toss the pasta to coat. Serve immediately.