1. FOR THE FISH: Adjust oven racks to middle and lower-middle positions and heat oven to 250°F. Pat fish dry with paper towels and season each fillet with 1/4 teaspoon salt; let sit at room temperature 20 minutes. Meanwhile, combine jalapeños and cornstarch in small bowl and toss to coat. Heat 1/2 cup of oil in 10-inch nonstick skillet over medium heat until shimmering. Shake excess cornstarch from jalapeños and add to skillet. Cook, stirring occasionally, until crisp but still pale, 1 1/2 to 2 1/2 minutes. Stir in garlic and continue to cook until garlic is just golden, 30 to 60 seconds. Pour contents of skillet through fine-mesh strainer into measuring cup; transfer jalapeños and garlic to ovensafe paper towel-lined plate and season with salt. Do not wash strainer.

2. Add oil to measuring cup to make 3/4 cup transfer to skillet (off heat). Place onion half in center of pan and let cool until oil registers about 180°F, 5 to 8 minutes. Arrange fish fillets, skinned-side up, around onion (oil should come roughly halfway up fillets). Spoon little oil over each fillet, cover skillet, transfer to middle rack, and cook for 15 minutes.

3. Remove skillet from oven and, using two spatulas, carefully flip fillets. Cover skillet, return to middle rack, and continue to bake until fish registers 130 to 135°F, 9 to 15 minutes longer, transferring plate with fried jalapeños and garlic to lower-middle rack halfway through cooking. Gently transfer fish to serving platter, reserving 1/2 cup oil, and tent loosely with aluminum foil; turn off oven.

4. FOR THE VINAIGRETTE: Combine jalapeños, shallot, cilantro sprigs, lime juice, and 1/2 teaspoon salt with reserved 1/2 cup fish cooking oil in blender. Blend on high speed until sauce is smooth, about 1 minute. Add any accumulated fish juice from platter, season with salt to taste, and blend 10 seconds longer. Strain vinaigrette through fine-mesh strainer, pressing on solids to extract as much liquid as possible. Discard solids.

5. To serve, pour vinaigrette around fish. Garnish each fillet with reserved crisped jalapeños and garlic, cilantro leaves, and avocado (if using). Serve immediately.

URL: http://www.cooksillustrated.com/recipes/detail.asp?docid=36345

Serves 4