Cut the steak into 1/8 inch strips. Sprinkle on all sides with salt and pepper.
Melt 3 tbs butter in a cast iron skillet until hot. Add steak and cook until browned, removed from pan.
Add the remaining 3 tbs butter, mushrooms and garlic, cook until the mushrooms have darkened, about 5 minutes.
Add the white ale and cook until the beer and butter has reduced by about half, about ten minutes.
Turn off heat, stir in the cream.
Add back in the steak and simmer until slightly thickened, about 8 minutes.
Season with rosemary, salt and pepper.
Serve over pasta.