A rich mushroom gravy to drown your mashed potatoes and stuffing in. I mean who likes gravyless potatoes anyways?
In a large skillet, melt butter. Add onion and mushrooms and saute for 3-4 min. until onions are fragrant and translucent. Add garlic and cook for 5 minutes. more until mushrooms have reduced and cooked off most of their water. Deglaze pan with the vinegars and tamari. Add the rest of the ingredients and stir until combined.
On medium heat, bring to a simmer. Simmer for 10-15 min. until the sauce has thickened and reduced. Season with salt and pepper as desired.