First make your hamburger, mix the mince, egg, worcestershire sauce, diced onion and salt and pepper together. And break into 3 patties, make your patties the same size as your focaccia. You can freeze two for latter use. Then turn your oven on 180 degrees to warm up. Now heat the olive oil and seal your burger on each side. Then place in oven. Now saute your mushrooms and onion with the butter this should only take a couple of minutes. Next cut your focaccia in half and I like to toast mine.