Melt butter in a large skillet over medium-high heat. Add onion; cook for 15 minutes or until well browned; set aside.
In a large bowl, combine turkey, 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, then partially cover, reduce heat to medium and cook another 5 minutes or until instant-read thermometer registers 160 degrees F.
While burgers are cooking, stir together blue cheese and mayonnaise in a small bowl. Place burgers on buns and top each with some of the onions, 1 tablespoon blue cheese sauce and a lettuce leaf. Makes 4 servings.
