Dice the red onion and puree or finely chop the chipotles.Dice the fish into 1-inch pieces. Pat fish dry and season with kosher salt. Heat oil in a large nonstick skillet over moderate-high heat. Add the onions and saute them for 1 minute; add the fish and fry for about 3 minutes until just cooked. Add the chipotle chiles, stir for 1 more minute and remove from heat.Fill taco shells halfway with fish mixture, then top with fruit salsa. Serve immediately.