Cut watermelon into ¼-inch pieces. Remove the rind of the oranges and cut the oranges into bite-sized pieces. Dice the red onion and slice fresh mint.
Add watermelon, oranges, red onion, and mint in a large bowl.
In a separate small bowl whisk olive oil, lemon, mustard, lemon juice, and zest until well combined. Add the dressing to the salad and toss to combine.