Sauté thinly sliced red onions and sliced pancetta in olive oil. Add chopped mushrooms (I used Swiss Brown, but you can use any other variant) and a bit of Sherry red wine vinegar. Sauté until mushrooms are browned; set aside. Toss romaine lettuce in a shallow bowl and add some olive oil and a little bit of Sherry red wine vinegar. Add the mushroom-pancetta-red onion mixture and top with crumbled blue cheese.