In a large pot of boiling salted water, cook the bucatini according to package instructions until al dente. Drain and set aside.

In a small bowl, mix the miso paste with a few tablespoons of warm water to create a smooth paste. Set aside.

In a large skillet, heat the olive oil over medium heat. Add the sliced mushrooms and sauté for about 5 minutes, until they are golden and tender.

Stir in the minced garlic and ginger, cooking for an additional minute until fragrant.

Pour in the vegetable broth and soy sauce. If you're using red pepper flakes, add them now. Bring the mixture to a simmer.

Stir the miso paste into the skillet until fully dissolved. Allow the sauce to simmer for another 5 minutes to thicken slightly.

Add the cooked bucatini to the skillet, tossing it in the sauce until evenly coated. Heat everything through for about 2 minutes.

Turn off the heat and plate the pasta. Garnish with sliced green onions, sesame seeds, and fresh herbs.