Mix all ingredients for vinaigrette in a jar/large measuring cup/dressing bottle and set aside.
Heat oil in a large skillet and saute mushrooms over medium heat. Add a bit of salt, minced ginger and garlic, and let sit over low heat for a few minutes. Stir in carrots, add a touch more salt with some pepper, too, and saute another 2 or 3 minutes. Add scallions and 3 Tbsp of the vinaigrette. Mix and turn off heat.
Cook soba noodles according to package directions (mine said 6 minutes). Drain and transfer to serving bowl. Pour in remaining vinaigrette, then mix in stir-fry. Garnish with sesame seeds.
Can be served anywhere from very warm to just-out-of-the-fridge cold. (You can eat it with a fox! You can eat it in a box!)