Watermelon Salad with Cotija, Jicama, and Lime

chopped red onion

lime juice (2-3 limes, depending on how big and juicy the limes are)

watermelon, rind removed, seeded and cubed into 1-inch pieces

crumbled cotija cheese

to 1 cup chopped peeled jicama

serrano chili pepper (or a quarter jalapeño), stem removed, seeded, minced

fresh cilantro, chopped

cumin, to taste