Mix the flour in a bowl with the Parmesan cheese, salt, pepper and dry mustard.

Stir in the melted butter so the mixture forms crumbs. Kneading with your hands, press dough into balls the size of small walnuts, (about a half tablespoon).

If the mixture is very dry and crumbly, add a bit more melted butter.

Set the balls on a buttered baking sheet and chill about 15 minutes.

Heat oven to 375 degrees F/190 degrees C.

Bake the cheese balls in the oven until browned, 35 to 45 minutes. Test for doneness by cutting into a ball; it should not be doughy in the center.

Transfer the cheese balls to a rack to cool. They can be stored in an airtight container for up to one week, or frozen for up to 3 months.