You could. I like the ground beef because it's so quick to cook. This would be perfect if you had leftover pot roast or tender stew meat you wanted to use up. If you used filet meat and added a mushroom mixture, it would be a take on beef wellington! Just know, no matter which meat you decide on for the filling, it's not going to bake for that long, so you'll want to have your filling fully cooked before you put it in the puff pastry and bake it.