Garlicky Shrimp-Cilantro Tacos (Tacos de Camarones al Mojo de Ajo)

vegetable oil

white onions, peeled and thinly sliced

or 3 serrano chiles, stemmed, seeded, and thinly sliced (see Notes)

butter

finely chopped garlic

shrimp (21 to 25 per lb.), peeled, deveined, and halved lengthwise

coarsely chopped fresh cilantro

Kosher salt

corn tortillas, warmed (5 to 6 in.; see Notes)

limes, cut into wedges