Whisk vinegar, mustard, salt and pepper in a small bowl. Whisking, drizzle in oil until well-combined (taste and add up to 1/4 tsp/1 mL granulated sugar if necessary).

Prepare watermelon, cucumbers and radishes and combine in a large bowl. Can be prepared to this point, covered and reserved in the refrigerator for up to 6 hours.

Drizzle dressing over salad and toss to combine just before serving. Place a Boston lettuce leaf on individual salad plates or line a large, deep platter with enough leaves to cover.

Spoon salad mixture into leaves and serve buffet style.

Makes 10 servings.

Tip: Add even more colour by using both yellow and red watermelon.

Make-Ahead: Can be made at least 8 hrs ahead.