Wash and cut the lettuce and chard leaves and place in a salad bowl. Peel the cucumber and cut into bite-sized pieces with the watermelon. Pluck and chop the peppermint leaves and add to the salad with a little lemon zest.
For the dressing, mix lemon juice and honey with the oil and season with salt and pepper to taste. Pour over the salad, stir and let steep for a moment.