Remove the woody stems from the mushrooms and cut the Portobellos into chunks about 1 inch square and the Shitakes in half. Heat 2 tablespoons of oil in a large skillet over medium-high heat until almost smoking hot. Add the mushrooms and cook for about 5 minutes or until they are browned and have released some liquid. Add a large pinch of salt and pepper and remove the mushrooms from the skillet and set aside.
Heat the remaining tablespoon of oil in the same pan, add the onions and cook until they soften and start to brown, about 5 minutes. Add the garlic and cook for another minute. Season with a little pinch of salt and pepper and stir to combine. Add the mushrooms back (along with any juices that have accumulated) into the pan then add the soy sauce and sherry or cognac or dry vermouth if you choose to use alcohol. Cook stirring until the liquid has evaporated. Add the vegetable broth and baby onions and cook for 3 or 4 minutes to heat the onions and reduce the liquid to about half. Lower the heat to low, add the vegan sour cream and heat through, stirring occasionally. Garnish with minced parsley. Serve over gluten-free pasta, rice sticks or steamed rice.