Rub the fillets with kosher salt and set aside. In a non-reactive bowl, combine the onion, lime juice, orange juice, jalapeno, sugar and tequila. Mix to dissolve the sugar, and add the fillets to the bowl. Marinate in the refrigerator for 2 hours, turning the fillets once after 1 hour. Remove the fillets from the marinade and set it aside. Pat the fillets dry with paper towels and lightly coat with the olive oil.
Heat a grill to high and place the fillets over direct heat until they are just cooked through - opaque at the center but still moist, approximately 3 to 4 minutes per side. While the fish is grilling, transfer the reserved marinade to a saucepan and heat until it is reduced to about 3/4 cup. Using tongs, remove the fillets to serving plates and divide sauce equally among them. Top with the cilantro.
Recipe courtesy of Alton Brown, 2007