Combine the buttermilk, red onion, scallions, dill, garlic, zest, and chile paste in a baking dish. Whisk or stir well.
Lay the fillets in the marinade, making sure you've got them completely coated. Cover with plastic and marinate in the refrigerator for at least one hour.
Remove the fish from the marinade and season it with salt and pepper. Coat the fillets well with flour.
Heat 1/4 inch of oil in a heavy skillet (cast-iron is best) until very hot but not smoking. Fry the fish in batches for about 1 1/2 minutes on the first side, then turn and fry for another 45 seconds. The crust should be golden.
Drain on paper towels and serve with the tartar sauce.