Prepare a charcoal or gas grill to medium-high.
Remove the pin bones from the trout fillet and season it well with sea salt and pepper. Generously brush the trout with olive oil—making sure to rub in the oil on the skin side.
Place the fillet, skin-side down, onto the medium-hot part of the grill. Let the fish cook for 2 minutes without moving it. Place a fish spatula under the fish, gently lift and move it—still skin-side down—to a cooler part of the grill. Continue to let the fish cook for 5 more minutes—checking periodically for charring on the skin.
Remove the fish from the grill and let rest for 2 minutes. Finish the fish with lemon juice and olive oil.
Season the berries and grapes with sea salt and drizzle with olive oil and white balsamic. Serve the trout with the berries and grapes. Serves 2 adults and 3 small grape lovers.