Sprinkle the trout fillets with salt. Set as many of fillets in a large skillet as will fit without crowding; if necessary, cook the fillets in two batches, reserving half of the sauce for the second batch.
Make the basting sauce by mixing together the ketchup, mayonnaise, olive oil, onion, brown sugar, mustard, Worcestershire sauce and cider vinegar.
Pour the sauce over the trout and cook over medium heat until the meat flakes easily with a fork, about 10 minutes. Use a fish spatula to carefully remove the trout to a platter.
