Preheat oven to 425 degrees.
In a 10" cast iron skillet ( or 2 smaller), melt butter over medium heat.
In a deep bowl, combine eggs, milk, flour, salt, cheese and rosemary. Using an immersion blender, process just until mixed. Overbeating will toughen pancake.
Pour batter into hot skillet and bake until puffed like a bowl and lightly browned, about 20 minutes. Less for 2 smaller pans.
Prepare filling while pancake bakes (or make in advance and reheat.)
Brown sausage and onions in a skillet with a bit of olive oil. When thoroughly cooked, remove and set aside. Add a bit more oil (or butter) and saute mushooms until they release their moisture and return sausage and onion mixture. Continue to cook adding the splash of sherry. (If making in advance, stop here.) Add cream and bring to a simmer for a creamy finish. If reheating, add cream right before serving.
Remove pan from the oven. If necessary, transfer to a serving platter. Pour filling into center of pancake. Top with additional chopped herbs if desired and serve.
Be prepared to share the recipe because it is delicious!