In a 10-inch cast iron skillet, heat 2 tablespoons of butter over medium-high heat. Season the steak with salt and pepper and sauté in a single layer until browned, then transfer to a clean plate. You may need to do this in 2-3 batches.
In the same skillet, melt the remaining butter and sauté the onions, garlic, and mushrooms until browned, about 8-10 minutes.
Meanwhile, in a food processor, blitz together the beef broth, coconut milk, wine, coconut aminos, nutritional yeast, Dijon mustard, ½ teaspoon of salt, and a pinch of pepper.
Return the beef to the skillet, pour the sauce over and let it simmer, until thickened and the steak and mushrooms have reached the desired tenderness. You may need to add more broth if the mixture gets dry.
Taste and adjust the seasonings. Garnish with fresh parsley and serve immediately with buttered gluten-free noodles or potato chips.