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1
Very thinly slice onion crosswise into rings with slicer. Set slicer aside. Heat vinegar, sugar, and 1/2 teaspoon salt in a small heavy nonreactive saucepan until sugar has dissolved. Remove from heat and stir in onion, then let stand until ready to use (about 25 minutes).
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2
Meanwhile, stir 1/4 cup lime juice together with cilantro, oil, and 1/2 teaspoon salt. Add fish and toss to coat, then marinate at room temperature 15 minutes.
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3
Whisk together mayonnaise, crema, 1/2 teaspoon salt, and remaining 3 tablespoons lime juice to make sauce.
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4
Preheat a gas grill with burners on high, covered, 10 minutes, then reduce heat to medium-high.
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5
Thread each piece of fish onto a skewer (discard marinade). Wrap exposed part of each skewer in heavy-duty foil to protect it from charring.
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6
Wrap tortillas in 2 foil packages and warm on grill, turning once, about 3 minutes total.
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7
Oil grill rack, then grill fish, covered, turning once, until opaque and just cooked through (for mahimahi), about 6 minutes total; or until just pink in center (for tuna), about 4 minutes.
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8
Very thinly slice radishes with slicer. Halve, pit, and peel avocados, then cut into 1/2-inch-thick slices. Drain onions.
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9
Serve fish with tortillas, crema sauce, onions, and remaining toppings.
