Whisk the olive oil, champagne vinegar, Dijon mustard, kosher salt, and black pepper together in a small bowl or jar until fully combined and slightly thickened.
3 tbsps (45 ml) extra virgin olive oil, 2 tbsps (30 ml) champagne vinegar, 1 tsp (5 ml) Dijon mustard, ½ tsp (3 g) kosher salt, ¼ tsp (1 g) black pepper
Pat the watermelon, cantaloupe, cucumber, tomato, and feta dry with paper towels to remove excess surface moisture before assembling.
1½ cups (225 g) seedless watermelon, 1½ cups (225 g) cantaloupe, 1 large English cucumber, 1 large heirloom tomato, 8 oz (226 g) block of feta cheese
Cut all ingredients into similar-sized cubes so the arrangement looks even and each bite stays balanced.
Arrange the watermelon, cantaloupe, cucumber, tomato, feta, and olives on a large platter in a loose mosaic pattern, alternating colors and leaving a small amount of space between pieces.
½ cup (75 g) Castelvetrano olives
Drizzle the dressing lightly over the top just before serving so the ingredients stay defined and the platter does not become watery.
Finish with coarse black pepper and scattered mint and basil leaves, then serve immediately.
coarsely ground black pepper, mint leaves, basil leaves