In a thick skillet, on medium heat, drizzle some olive oil, add the shredded leeks, stir until they soften. Add a handful of button mushroom, or a portobello cut into medium size pieces, add 1 tablespoon of butter. Add 4 portobello mushrooms to the skillet cap down until they slightly brown, drizzle balsamic vinegar on the gills and sprinkle with oregano.