1. Heat oil in a large frying pan over high heat. Add onions and chiles and cook, stirring frequently, until onions are lightly browned, about 10 minutes. Pour vegetables into a bowl and set aside.
2. Melt butter in same frying pan over medium-high heat, add garlic, and stir until sizzling, about 1 minute. Add half of shrimp to pan. Stir often until shrimp are bright pink and no longer wet-looking in thickest part (cut to test), about 5 minutes; transfer shrimp to bowl with reserved vegetables. Add remaining shrimp to pan and cook as above, then return cooked shrimp and vegetables to pan and stir everything together until hot, 2 to 3 minutes. Add cilantro and salt to taste.
3. Serve shrimp in a large bowl with tortillas (use 2 tortillas, stacked, per taco), accompanied by lime wedges to squeeze over top.
Note: Nutritional analysis is per taco.