In a large bowl, carefully combine the meats, steak sauce, egg yolks, salt and pepper, being careful not to overwork the mixture. Form into 4 patties, slightly wider than the buns.
Preheat a grill over medium-high heat, or heat a cast iron skillet over medium-high heat.
Cook the patties for 4 minutes on one side, then flip over and cook for an additional 3-4 minutes (for medium doneness). When there is a minute left, add 1 tablespoon of blue cheese to the top of each patty. When done, remove to a plate and cover loosely with foil.
Place the buns on the grill or in the skillet and cook until the cut sides are toasted.
To assemble, spread 1 tablespoon of blue cheese dressing on the bottom bun halves, then place the patty on top. Top with lettuce, tomatoes, onions and the top bun and serve.
Notes
Adapted from Barefoot Contessa at HomeNutrition information provided as an estimate only. Various brands and products can change the counts. Any nutritional information should only be used as a general guideline.