Most restaurants are more than happy to accommodate any restrictions you have. It is recommended that you call ahead! Remember that these places have a set menu, with set recipes. It takes time to deviate from these recipes and get a good result. Chef's are artists at heart, and do not want to serve you sub par food. If you call and let them know a general time that you plan on eating at their restaurant, they will ensure that the food you need is available to you in a timely manner. This also ensures that you will not get food that will cause you physical harm.
Are all restaurants willing to do this? Sadly, no. But it never hurts to call and ask. This will make things much more pleasant for you. A simple phone call can increase the quality of your food and your time.