Preheat oven to 190 degrees C. Grease a baking tray with cooking spray.
Combine the chipotle chilli, adobo sauce, salsa and 1 tablespoon lime juice in a blender and blend until smooth; brush the mixture over both sides of each tilapia fillet.
Spread the crushed corn chips into the bottom of a deep dish and coat the fillets with the crumbs. Arrange the fillets on the prepared baking pan.
Bake the fillets 15 minutes; turn and lightly spray with cooking spray. Continue to bake until golden brown, another 10 to 15 minutes.
While the fish bakes, place the avocado, sour cream and 1 tablespoon lime juice in a blender and blend until smooth. Stir in the milk 1 tablespoon at a time until the consistency is similar to ranch dressing. Season with salt and pepper. Spoon the sauce over the baked fillets to serve.