Preheat oven to 425 degrees F. Melt the butter in a 10-inch cast iron skillet.
Meanwhile, add the eggs, flour, milk, salt, and pepper to a blender in that order. Blend until very well combined.
Once the oven is preheated, pour the batter over the melted butter in the skillet. Sprinkle on the mushrooms and sausage.
Bake in preheated oven for 12 minutes. Sprinkle on the cheese and bake for an additional 8-10 minutes, or until puffy and deep golden.
Remove from oven and sprinkle with the chives. Serve hot with grass-fed butter.