In a small mixing bowl, whisk together the miso paste and butter until it comes together to form a smooth, fluffy paste. In a same pan where the mushrooms are, add the minced garlic and a tablespoon of olive oil, and stir-fry over medium heat for 30 seconds. Add in the miso-butter mixture and heavy cream, and bring this sauce to a light boil, stirring gently. Finally, add the cooked pasta into the sauce, and stir until the pasta is well coated. Season with salt and pepper to taste, and let cook for 1 to 2 minutes until the pasta is al dente, then remove from the heat.