In a pot of boiling water, cook the hibiscus flowers for 20 minutes. Strain and set aside. For the dressing, in a bowl, whisk together the olive oil, vinegar, mustard and garlic. Thin with a little water if necessary. Season with salt and pepper. Set aside. In a bowl, mix the tomatoes, cilantro leaves, lettuces and onions with the drained hibiscus flowers. Add some dressing and mix again. Season with salt and pepper.
Hibiscus flowers bring a nice acidity to this salad.