When you're going to a restaurant, pull up the menu and plan your meal AHEAD of schedule. When you get to the restaurant, decline a menu. When the waitperson takes orders, give yours first and excuse yourself to the restroom to wash your hands while others are ordering to avoid changing your order.
When you place your order, make certain to give specific preparation instructions to the waitperson (i.e., no butter, seasoning, extras of any kind, etc.). Have your meal split in half before it comes to you. Half plated to the table and half in a to-go container.