Add a drizzle of olive oil or a splash of water to a large skillet over medium flame. Add the sliced onion with a pinch of sea salt and saute until caramelized, about 45mins.
Keep a close eye on them and stir often so they won’t burn. You can help them along with an extra splash of water and a drizzle of maple syrup or agave.
Add the mushroom to the pan with the caramelized onions and saute together a few more minutes until the mushrooms have wilted. Taste and adjust seasonings with sea salt. Set aside and keep warm.