Cook Pasta: Bring a pot of water to a boil and cook pasta according to package instructions for al dente**. Reserve about ½ cup of the pasta water, then drain the pasta and rinse under cold water. Set aside.
Cook Onions: Heat butter in a pan over medium heat. Once the pan is hot and butter melts, add onions and cook until onions start to brown, about 4 minutes. Stir occasionally so the onions don't stick to the pan.
Add Garlic: Add garlic and stir for 30-60 seconds until fragrant.
Add Mushrooms: Add mushrooms and cook until tender, stirring occasionally.
Add Spinach (optional): If using spinach, add it in now and stir until wilted (or thawed if using frozen spinach).
Add Pasta and Soy Sauce: Stir in cooked pasta, then drizzle soy sauce over and stir well to combine.
Add nutritional yeast: Stir in nutritional yeast.
Add Pasta Water: Add in about ¼ cup pasta water and toss to combine. If pasta still looks dry, add in more pasta water. The pasta water should just lightly coat the pasta so it's not dry.
Season: Turn off heat. Sprinkle with pepper and lemon juice (if using). Taste and add salt/pepper/lemon juice to taste. Garnish with nori strips right before serving.
Notes
Mushrooms: Use a mix of mushrooms for best flavor. I recommend Asian mushrooms like shimeji, shiitake, maitake, enoki, oyster, and king oyster mushrooms. My favorites to use in this pasta are shimeji, enoki, and shiitake. Cooking Pasta: Use the lower amount of time indicated on your pasta packaging. For example, if it says to cook for 8-10 minutes, cook for 8 minutes since the pasta will cook a bit more in the pan with the rest of the ingredients.