Heat the olive oil in a skillet over medium-low heat. Add the sliced onions. Cook for 30-40 minutes, stirring occasionally, until the onions are deeply golden brown and caramelized. If the onions start to stick, add a splash of water.
In a bowl, mix together the ground beef, salt, pepper, and Worcestershire sauce. Form into four thin, oval-shaped patties.
Cook the patties in the same skillet over medium heat until browned, about 2-3 minutes per side. Remove and set aside.