In a medium bowl, whisk together the juice from one lime with the oil, half of the sliced onions and half of the cilantro. Marinate the fish in the mixture for 15-20 minutes (marinating for too long will start to cook the fish in the lime juice).
Preheat your grill or grill pan on high heat. Grill the fish for about two minutes per side, then lightly season with salt.
Place the tortillas on the grill for about 20 seconds per side.
Divide the fish among the tortillas and garnish with shredded cabbage, cilantro, red onion slices, avocado, sriracha mayo and lime slices.
Slowly add 1 teaspoon of sriracha to the mayonnaise while whisking. Adjust to your taste.