While onions cook, mix ground beef, crushed boullion cube, worcestershire sauce, ketchup, mustard, salt, and pepper. Form into 4 large patties. Heat a cast-iron pan over medium high heat with a large drizzle of olive oil. Once hot, add patties and cook for 2 minutes or until bottom started to blacken. Flip patties and cook for 1-2 more minutes or until burgers reach desired doneness. Turn off heat.