While the pasta water is still hot, add the miso to a cup of pasta water and whisk until all the miso paste has dissolved.
Set a large skillet over medium heat and add the butter. Once the butter has melted, add the minced garlic and sauté for about 30 seconds. Then add in the sautéed mushrooms, miso-pasta water and heavy cream. Bring the sauce to a gentle boil and then reduce the heat to low. Simmer the sauce for about 2 minutes until it thickens.
Then turn off the heat and add the grated pecorino. Stir until the pecorino is melted into the sauce.
Then add the cooked bucatini to the sauce and stir until the pasta is well coated. TIP: The sauce may thicken further once you add the bucatini. If so, add additional pasta water to thin out the sauce.
Plate the pasta and garnish with grated parmesan and scallions. Serve immediately. Enjoy!