Peel and finely chop the onion. Drain bacon in a pot, add onion and fry, add lentils, deglaze with 200 ml water and vinegar. Simmer lentil vinaigrette for 15-20 minutes, season with salt, pepper and sugar. In the meantime, clean and wash the romaine lettuce, put a few leaves aside for serving and cut the remaining romaine lettuce into strips. Wash, clean and slice the mushrooms. Clean, wash and slice the tomatoes. Mix the romaine lettuce, mushrooms and tomatoes, arrange on plates with the lettuce leaves set aside, spread the lentil vinaigrette on top and serve garnished with parsley and coloured pepper to taste