A classic burger consists of a beef patty, some salad/ tomato/ onion, a burger bun and some sauces. That already has all the components of a well balanced reasonably healthy meal but a typical burger bun doesn't provide much starch. The classic solution is to take an order of fries on the side. That works but I'm looking for something I can add as an extra layer to the burger.
My first thought was to make a double burger with a beef patty and a vegan burger patty but the vegan patties are usually high in protein because they try to imitate the meat.
What are good options to add as an extra layer in my burger to make the burger a more balanced meal?