Light the grill or heat the broiler. In a small stainless-steel saucepan, combine the oil, garlic, sage, and rosemary. Cook over moderately low heat until the garlic just starts to brown, about 2 minutes. Remove from the heat and immediately stir in the vinegar, 1/4 teaspoon of the salt, and the pepper.
Put the trout fillets in a medium glass dish or stainless-steel pan. Sprinkle the fish with the remaining 1/4 teaspoon salt. Add half of the oil-and-vinegar mixture and turn to coat. Grill or broil the fish skin-side down for 2 minutes. Turn and cook until just done, about 2 minutes longer for 1/4-inch-thick fillets. To serve,whisk the remaining oil-and-vinegar mixture and pour it over the hot fish.