Heat a large skillet over medium-high heat for about 2 minutes. Add the mixed mushrooms to the dry skillet, sauté for 5 minutes until slightly charred and fragrant. Drizzle in olive oil, season with salt and cracked black pepper, and sauté for an additional minute. Remove the mushrooms and set aside, keeping juices in the pan.

Bring salted water to a rolling boil in a large pot. Add the dried bucatini and cook until al dente, about 8-10 minutes. Reserve at least 2 cups of starchy pasta water before draining.

In the same skillet, melt 2 tablespoons unsalted butter over medium heat. Add minced garlic, sauté for 30 seconds. Stir in cooked mushrooms, heavy cream, and miso dissolved in 1 cup reserved pasta water. Increase heat to a gentle boil, reduce to a simmer for 5 minutes until sauce thickens.

Add cooked bucatini directly into the mushroom mixture in the skillet. Sprinkle in Pecorino Romano cheese and gently toss until cheese melts and pasta is well-coated. Add more reserved pasta water if sauce is too thick.

Plate the Miso Mushroom Bucatini, scoop up plenty of sauce. Garnish with freshly sliced scallions and extra Pecorino Romano or cracked black pepper. Serve warm.