Cut the mushrooms into large slices. Scald tomatoes, peel and core them and cut into wedges. Dice the onion. Chop the garlic and herbs. Heat 2 teaspoons of oil, sauté the onion and garlic in it, add the mushrooms and fry over a medium heat for about 3 minutes. Mix in the herbs and season with salt and pepper.
Mix the vinegar with the broth, mustard and the remaining oil and season with salt, pepper and sweetener. Spread the lettuce on the plate, arrange the tomatoes and warm mushrooms on top, drizzle with the vinaigrette and serve with the baguette.