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Adjust rack to middle position and preheat oven to 425 degrees. Line a baking sheet with foil and lightly oil (or spray with nonstick cooking spray). Wash and dry produce.
Halve, peel, and finely chop half the onion (whole onion for 4 servings). Zest and quarter lime (for 4, zest one lime and quarter both).
In a small bowl (medium bowl for 4), combine 1½ TBSP chopped onion (3 TBSP for 4), juice from half the lime (whole lime for 4), and a pinch of salt.
In a separate small bowl, combine Mexican cheese and Monterey Jack.