Preheat the oven to 375 degrees . Grease an 8-by-10-inch baking dish. In a medium bowl, combine the green chiles, onion, butter, chili powder and salt.
On a work surface, lay the flounder fillets skin side up and spread a heaping teaspoon of the chile butter on each fillet; sprinkle with about 2 tablespoons crushed tortilla chips. Starting with the narrowest end, roll up each fillet. Place seam side down in the prepared baking dish, and top with the remaining tortilla chips.
Bake the fish fillets until firm and opaque, about 25 minutes. Serve topped with the remaining chile butter.