YouTube user aVeganCookingShow shows how to prepare a portobello mushroom and mashed potatoes. First, he makes the mushroom marinade. Chop four cloves of garlic and a quarter onion. Combine these in a bowl with a quarter cup of olive oil and a quarter cup of balsamic vinegar. Remove the stems of three portobello mushrooms and place the caps in the marinade and let it sit for about an hour. Roast a clove of garlic for the potatoes by cutting off the top of a clove and pouring olive oil through the top. Wrap it in foil and put it in the oven at 450 degrees for half an hour. Then wash eight potatoes, cut and quarter them. Put them in a pot of water and boil for about 10 minutes with two chopped cloves of garlic. Remove the water, mash the potatoes with 3/4 cup of margarine, mix in 1/2 cup of soy milk and mash in the roasted garlic. Top off with steak seasoning. Grill or sautv© the marinated mushroom heads for 3-4 minutes. Prepare the mushrooms and potatoes together.