Add the mushroom, cook until browned and set aside.
Add the ground beef and onions.
Sautè until the beef is cooked through.
Drain the beef fat (I use a colander to do this, not because it's unhealthy but because we want a different taste).
Return the mushrooms, onions and beef to the pan.
Add the garlic, broth and sour cream.
Bring to a simmer and sprinkle the arrowroot over top. Then stir to thicken.
Add a sprinkle of oregano. Not too much, it just lends a really nice flavor.
Serve over mashed potatoes or mash cauliflower.
Notes
If you're strict paleo, dairy-free or Whole30, make it with coconut cream or cashew cream instead of sour cream.
I used a lactose-free sour cream, which I find easier on digestion. But use what you like!
Arrowroot should be added at the end of cooking because it thickens quickly and can start to break back down and become watery if left over heat for an extended period of time