Heat a nonstick skillet over medium heat and spray with olive oil. Add the sliced onions and cook for 5–7 minutes, stirring often, until softened and lightly caramelized. Remove and set aside.
Season the ground beef with salt, pepper, and garlic powder, then shape into a thin patty roughly the size of the bread slices.
In the same skillet, cook the beef patty for 3–4 minutes per side, or until fully cooked. Remove from the skillet.
Spray one side of each slice of bread with olive oil. Place one slice of bread, oil side down, into the skillet. Top with the beef patty, Swiss cheese, and caramelized onions.
Top with the second slice of bread, oil side up, and cook over medium-low heat for 2–3 minutes per side, until the bread is golden and the cheese is melted.
Slice in half and serve warm.