Cook the pasta in heavily salted water, reserving one cup of pasta water before draining. Be sure to check the package instructions for the exact al dente cooking time, but this should take about 7-9 minutes.
Add the oil and chili flakes to a Dutch oven over medium heat.
Add the onion and cook until translucent (about 3-5 minutes), then add the mushrooms and garlic. (Tip: add an extra splash of oil if necessary)
Cook until the mushrooms start to soften, then transfer them to a bowl.
Add the vegan butter to the dutch oven. Once melted, add the cooked pasta.
Add the miso to the pasta and toss to combine with a splash of pasta water. Add the mushrooms back to the pot, alongside the vegan cheese and a very generous pinch of pepper to taste.
Add more pasta water to emulsify the sauce, then season with salt to taste and garnish with more pepper and vegan cheese when plating.