Using a crinkle potato grater, run the grater down each side of every watermelon slice to produce crinkles. Then, cut the slices into one-inch sticks. (OR skip the crinkle step and simply cut watermelon into sticks like home fries without the crinkles.)
Zest the orange and lemon. You will need one teaspoon each of zest. Place fresh zest, yogurt, condensed milk in a bowl. Pour in 1 tbsp. of squeezed orange juice from the orange, and sightly whisk the dip with a whisk.
Drizzle the watermelon sticks with an additional 1 tbsp. orange juice and 1 tbsp. lemon juice. Leave for 5 minutes to marinate.
Serve the watermelon fries on a platter with the bowl of yogurt dip. Garnish with mint or cilantro, if desired. Season fries with tajin, salt, or chili powder, if desired.
Notes
Watermelon fries and yogurt dip can be prepared the day before and kept refrigerated in separate air-tight containers. Any leftovers should be refrigerated and stored separately.