The whole point of the ground flax is to increase the nutrition of the tuna while lowering the carbs, but if you don't have it go ahead and use breadcrumbs or even crushed crackers. Another option to keep it lower carb is to add almond meal instead of the ground flax. All these would use the same 3 tablespoons. That said, you can just leave the flaxseed out - the egg will do the job of holding the patties together.