Cook pasta in well salted water according to package instructions. Reserve at least ½ cup pasta water. I usually reserve 1 cup just in case more is needed.
Heat a tall-walled skillet/saute pan over medium-high heat. A Dutch oven or medium stock pot also works. Once hot, add butter.
Once melted, add the sliced mushrooms and diced shallot. Sauté until mushrooms are softened and golden, about 8 minutes.
In a small bowl, whisk miso with 1 tablespoon of water to thin. Add in the miso, minced garlic, soy sauce and optional red pepper flakes. Cook another 1-2 minutes, until aromatic.
Sprinkle in all-purpose flour. Sauté another minute to "cook" the flour.
Turn heat down to medium-low. Stir in 1 teaspoon lemon zest. Slowly whisk in the half and half and ½ cup of pasta water. Simmer over medium-low heat until the pasta sauce is thickened enough to coat the back of a spoon without sliding right off. Add extra pasta water as needed to thin the sauce.
Add the cooked pasta directly to the pasta sauce or serve individual portions topped with the mushroom sauce. Garnish each portion with lemon wedges, ample grated Parmesan and salt and pepper to taste.
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Notes
Mushrooms: Half baby bella/shiitake is also a good combo. White mushrooms work but won't be as flavorful. Wild mushrooms are delicious if you can get your hands on them.
Leftovers: This recipe is best enjoyed immediately, but it will keep 3-4 days in the fridge. Add a splash of vegetable broth or half and half before reheating individual servings in the microwave or on the stove-top over medium-low.