In a large mixing bowl, whisk the egg, Red Boat, mustard salt, and pepper. Add ground beef, ground sausage, garlic and onions and mix well. Divide into 4 parts and Form into oval patties that look like steaks.
Heat oil and butter in a large skillet. Add the steaks and cook for 8 minutes on one side before flipping over. Then cook for another 8 minutes until they are cooked through with an internal temperature of 160 ℉. Transfer steaks to a plate and cover to keep warm.
Wipe the skillet clean.
For gravy: Heat 1 tablespoon of butter in the same skillet. Add mushrooms and onions. Sprinkle with rosemary and season with salt. Cook about 5 minutes until brown.
In a small bowl, make a slurry by mixing together 2 tablespoons of broth with tapioca flour.
Add 2 cups of broth, mustard and Red Boat to the mushrooms and onions.
Stir in the slurry, heat to just bubbling. Turn down to a simmer and whisk until thickened about 1-2 minutes.
Add salt and pepper to taste.
Return the Salisbury steaks to the skillet and spoon the onions and mushrooms over the steaks. Simmer for a few minutes. Garnish with parsley and serve.