Melt butter in a large skillet over medium-high heat. Add onion and cook 15 minutes or until well browned; set aside.
In a large bowl, combine chicken, ricotta, Worcestershire sauce, mustard, salt and pepper. Stir until well combined. Form into 4 equal-size patties and flatten.
Heat oil in a large nonstick skillet over medium heat. Cook burgers about 5 minutes per side. Partially cover, reduce heat to medium and cook another 5 minutes or until instant-read thermometer registers 160° F.
Place burgers on buns and top each with some of the onions and a lettuce leaf.