Start a pot of water for the soba noodles. Do not salt - not needed for soba noodles.
In a large skillet or cast iron, heat 1 tablespoon of sesame oil.
Add sliced mushrooms and saute for 5-6 minutes stirring often.
Meanwhile, mix miso, vinegar, remaining sesame oil, and soy sauce in a small pyrek with a pour spout or a small bowl. Whisk until smooth.
Once mushrooms have softened, add garlic and saute for another minute.
Meanwhile, once water has reached a boil, add soba noodles and set a timer for 4 minutes.
Meanwhile, add spinach, half at first, and turn with tongs wiht mushrooms. Adding the second half after the first has started to wilt. Once it all has wilted, add miso sauce and stir well to combine and simmer 2-3 minutes.
After 4 minutes of boiling, drain the noddles and then add noodles, a tongful at a time, mixing into sauce after each addition.
Stir well to combine all noodles with sauce.
Top with scallions, peanuts, and sesame seeds.