Fill a medium pot with water and add a generous amount of salt. Put it over high heat. Keep an eye on this pot and begin your sauce in the meantime.
While the pasta water comes to a boil, clean and slice the mushrooms, peel and mince the garlic, and peel and dice the onion.
Add the olive oil to a skillet over medium high heat. Add the onion, stirring occassionally, for about 5-7 minutes.
Add the mushrooms and a sprinkle of salt. Stirring occasionally, cook until the mushrooms are golden and soft.
Add the minced garlic and stir, cooking for a couple of mintues. Do not let the garlic burn.
When the pasta water comes to a full boil, add your pasta and cook according to the package instructions. When it comes time to drain the pasta, reserve 1/2 cup of the pasta water.
Once the mushrooms are completely cooked, add the pumpkin and stir to completely combine what is in the pan.
Add the tahini, salt, pepper, and dried herbs. Add the nutritional yeast if you are using it. Stire everything to combine.
Take your reserved pasta water and add it little by little, constantly stirring, until the sauce is the consistency you prefer. If you want to add fresh herbs or grated cheese, now is the time.
Remove the skillet from the heat.
Add the spinach leaves to your hot sauce, stirring them in to combine. They will wilt in the residual heat of this sauce.
Toss your pasta with the sauce, making sure to really combine everything so each serving has mushrooms and spinach. Serve immediately with the garnish of your choice.
