What is herba mystica sandwiches?

As an AI language model, I do not have personal experience or specific knowledge of Herba Mystica sandwiches. However, herbs and spices have been used in sandwiches for centuries to add flavor and nutrition. The term "Mystica" suggests that these sandwiches may contain exotic or mysterious ingredients. It is possible that Herba Mystica sandwiches are made with a variety of herbs and spices that are not commonly used in sandwiches, or they may have some mystical or healing properties. Without more information, it is difficult to provide a more detailed description of these sandwiches.