Preheat the oven so it reaches an even temperature before baking.

Slice the tops off the onions and hollow them out carefully with a spoon, keeping the outer layers intact. Set the removed onion centers aside.

Arrange the hollowed onions upright in a baking dish so they stay stable during baking.

Melt the butter in a skillet with part of the olive oil. Add the chopped onion centers and cook over medium heat. Stir occasionally until they caramelize and turn golden brown.

In a large bowl, combine the ground beef, chicken hamburger seasoning, garlic powder, black pepper, and Worcestershire sauce. Mix gently.

Fold a portion of the caramelized onions into the beef mixture to add sweetness and depth.

Spoon the mixture into each hollowed onion, pressing lightly so it fills the space without overpacking.

Drizzle the remaining olive oil over the tops of the onions.

Cover the baking dish loosely with foil so the onions steam lightly and begin softening in the oven.

Bake until the onions start to become tender.

Remove the foil and sprinkle half of the shredded Gruyere over the filled onions. Continue baking so the cheese melts and browns slightly.

Add the remaining Gruyere on top and bake until fully melted and the onions are completely tender but still holding their shape.

Remove from the oven and allow them to rest briefly before serving.