How to Make It
Roast the jalapeños directly over a gas flame or under the broiler, turning, until charred all over, 2 to 3 minutes. Transfer to a bowl, cover tightly with plastic wrap and let cool. Peel, stem and seed the jalapeños, then finely chop.
In a medium bowl, whisk the buttermilk with the mayonnaise, sour cream, garlic and onion powders, celery salt, lemon juice and a dash of hot sauce. Whisk in the chiles and season the jalapeño-ranch dressing with salt and pepper.
In a medium bowl, whisk 1 tablespoon of the olive oil with the vinegar. Add the cucumber, tomato and dill and toss to combine. Season with salt.
In a mini food processor, pulse the panko until finely ground. Spread in a shallow bowl. In a second shallow bowl, whisk the milk with the egg. In a third shallow bowl, spread the flour. Season the fish with salt and pepper, then dredge in the flour and shake off the excess. Dip the fish in the milk-egg mixture, then dredge gently in the panko.
In a large, deep skillet or saucepan, heat the remaining 1/2 cup of olive oil. Fry the fish over moderately high heat, turning once, until golden brown, about 10 minutes. Transfer to a paper towel–lined plate to drain and season with salt.
Spread some of the ranch dressing on the bottom buns. Top with a piece of fish and the green tomato–cucumber relish. Drizzle with more dressing, close the sandwiches and serve, passing any remaining ranch dressing at the table.