Cook pasta in heavily salted water. Reserve 1 cup for pasta water before you drain. I use gluten-free pasta so I rinse after cooking. If you are using normal pasta no need to drain.
Add oil and chili flakes to a Dutch oven. Add onion and cook until translucent. Add mushrooms and garlic. Add a splash of more oil if necessary. Cook until mushrooms start to soften. Transfer them to a bowl. Add butter into the pot. Add cooked pasta to the melted butter. Add miso to pasta and toss to combine with a splash of pasta water. Add mushrooms back to pasta and add cheese and tons of pepper. Add more pasta water to emulsify the sauce. Add salt to taste and garnish with more pepper and cheese when plating.
*Macros are provided as an estimate for convenience. Your final numbers will vary depending on ingredient brands, substitutions, and how you portion the recipe.