The salsa that goes with these tostadas is powerfully sweet and tangy but won't drown out the delicate-flavored white fish. Try pairing the sauce with chicken or pork as well.
Position rack in upper third of oven; preheat to 400 degrees F. Brush both sides of tortillas with 1 tablespoon oil and arrange on a baking sheet. (It's OK if they overlap a bit; they will shrink as they cook.) Bake, turning once, until browned and crisp, 16 to 18 minutes. Transfer to a wire rack and let cool.
Meanwhile, segment oranges: Using a sharp knife, slice both ends off and remove the peel and white pith. Cut the orange segments from their surrounding membranes. Combine the segments, onion, cilantro and salt in a medium bowl.
Sprinkle fish with chile-lime seasoning (or rub). Heat the remaining 2 tablespoons oil in a large nonstick skillet over medium-high heat. Add the fish and cook, turning once, until it flakes with a fork, 3 to 5 minutes per side.
Flake the fish into chunks. Spread avocado on the tortillas and top with lettuce, the fish and the salsa.