Heat butter or ghee to medium-high heat in a large skillet.
Season the steak with salt and pepper.
Add the meat to the pan, and cook for 3 to 4 minutes, until browned. Remove from the pan and allow to rest.
Add the onion, mushrooms, garlic, thyme, salt, and pepper to the pan. Cook for 5 minutes, until the onion has softened.
Reduce heat to medium.
Add broth, cream cheese, coconut aminos, and Dijon mustard and cook for 5 to 8 minutes, until thickened. Whisk as needed to help the cream cheese melt.
Return the beef to the pan, and simmer for 5 more minutes.
Remove from heat and stir in sour cream.