Preheat grill to medium high heat. Prep your hamburger patties by adding soy sauce and black pepper to ground beef and gently mix to incorporate.
Form four 1/3 lb patties and place in the refrigerator until ready to grill.
Place a cast iron pan over direct heat on your grill grate. Add the beef tallow until its melted and hot.
Add the sliced onions and sauté for 3 to 4 minutes until the begin to turn translucent.
Add the sliced mushroom and salt to taste and sauté until all liquid from the pan has evaporated and mushroom develop a golden brown color which will take 6-7 more minutes.
Add the flour and mustard powder until thoroughly combined, about 1-2 minutes.
Slowly pour in your beef broth and sour cream and continuously stir until it begins to simmer.
Remove the pan from direct heat, but close enough to stay hot.
Let your charcoal heat up to high heat and place your burgers on the grill.
Grill for 3 minutes per side until you have developed a nice crust and the internal temp hits about 130 degrees. Remove the burgers from the grill and place each patty on top of your stroganoff to rest.
Add a thin layer of butter to all 3 hamburger buns and grill until they are toasted.
Immediately before serving flip each hamburger patty and layer bottom bun, hamburger patty, extra gravy, bun, patty, gravy, chive or parsley and finally your top bun.
These will no doubt make you feel better when the weather turns for the worse. Enjoy!