Trim and discard ends of zucchini. Cut zucchini into 3- to 4-in. lengths. Cut each length into ¼- to ½-in.-thick matchsticks; set aside. Peel carrots and cut same way as zucchini; set aside. Peel jicama and cut in half. Set halves, cut side down, on a cutting board and cut into ¼- to ½-in.-thick slices. Stack 3 or 4 slices and cut into ¼- to ½-in.-thick matchsticks; set aside.

In a large frying pan over high heat, toast cumin seeds (if using) until fragrant, 2 to 3 minutes. Add vegetables. Cook, stirring, until sizzling. Add salt and juice and cook until vegetables are just tender, about 5 minutes. Sprinkle with pepper and serve hot, warm, or at room temperature.