Clean and slice the mushrooms and slice the scallions. Get them started in a skillet with a little oil (a neutral one like canola) and minced garlic and ginger.
Bring a pot of water to boil and add the soba noodles. They cook quickly (and couple of minutes) so as soon as they go in, fill a large bowl with cold water and ice cubes to be ready to transfer the noodles into.
Keeping an eye on the veggies, seasoning to taste and giving them a good stir, allow them to continue to cook while the noodles finish cooking, cooling in the ice bath, and while you make your sauce.
To make the sauce: start with a heaping spoonful of peanut butter. Add about a tablespoon of the noodle cooking water or the ice bath water and whisk to thin it out. Add a little more if you want it thinner. The sauce is made entirely by using your senses, so the only thing to remember is to add things slowly, taste, and adjust.
To the peanut butter, add a small drizzle of sesame oil, a dash or two of the fish sauce, a few dashes of rice vinegar and a splash of soy sauce. Whisk, taste and adjust until you’re happy with the flavor of the sauce. Add some sriracha if you like heat.
Drain the noodles from the ice bath and toss them in the sauce. Add the veggies and mix well to incorporate. Then serve. Garnish with more scallion, thinly sliced, some sesame seeds, crushed peanuts, or other veggies like carrot ribbons or radish matchsticks. Enjoy!