Wash the trouts in cold water. Dry them inside and out. Sprinkle with lemon juice and gently salt them.
Brush 4 sheets of aluminum foil with oil. Wash dill and shake off the water, cut it coarsely. Peel the onion, cut it finely and mix in dill. Fill the trout with the mixture and close them.
Place the fish on the foil sheets. Bring the ends of the foil together and wrap them in tight packs. Grill 15 minutes on each side. Place the Trouts in the dishes and decorate them with lemon slices.