Lightly oil the grate of an outdoor grill. Arrange lemon slices on grate so they overlap slightly and form a shape slightly larger than your trout fillet.

Preheat the grill for medium heat to 350 degrees F (175 degrees C).

Heat olive oil in a small skillet over medium-high heat.

Add garlic and onion to skillet; sauté until garlic browns slightly, 2 to 3 minutes. Season with salt and black pepper; squeeze reserved lemon ends over top.

Gently lay trout fillet on lemons, skin-side-down. Brush garlic mixture over top of fillet.

Cook trout until flesh flakes easily with a fork, about 10 minutes.