Cook the pasta per package instructions. Reserve ½ cup of the pasta water before draining.
Add the oil to a frying pan and bring the heat to medium-high. Put the garlic and mushrooms into the pan and cook for 10 minutes, stirring frequently.
Put the butter, miso paste, soy sauce, coconut milk, nutritional yeast, and salt into a small pot and cook on low heat for 5 minutes until everything is melted and combined.
Add the cooked pasta to the frying pan, along with the reserved pasta water.
Add the miso sauce mixture, spinach, and lemon juice to the pan.
Warm everything through on low heat while stirring for a few minutes until the spinach has wilted.