Peel and thinly slice the garlic cloves. Place ½ in a mixing bowl and the other half in the pressure cooker. Peel the onion and slice in half. Cut one half into small cubes and the other into thin slices. Put the thin sliced version to one side to use later and the other half into the mixing bowl.
Add to your current bowl (that has the onion and garlic in) the rest of the Salisbury steak burger ingredients and mix well. Make into 4 burger shapes.
Add half the butter to the pressure cooker and set the pressure cooker to sauté. As it warms up mix the garlic and onion into the butter and place the burgers on top. Cook for approximately 2 minutes on each side or until you have a crispy coating on the burgers. Remove the burgers with a spatula and put to one side.
Slice the mushrooms into quarters and add them into the pressure cooker along with the rest of the butter, mustard powder, cayenne pepper and salt and pepper. Mix well and be careful of it spitting at you.
Time to cook. When the mushrooms have gone soft, add in your mushroom bone broth, stir and cancel the sauté setting on the pressure cooker. Place the raw onions on top and then the burgers on top of the onions and cook for 5 minutes on manual pressure. Use quick pressure release and remove the burgers. Use coconut milk to thicken the mushroom gravy and serve while still warm.