I want to clarify what seems to be a misconception about signature dishes or just clarify it to eliminate some questions.
Signature dishes are recipes you have saved in your cookbook and then cooked 1000 times. That is 1000 times AFTER you save it. Signature dishes have the chance to enhance the buffs on a cooked item. That is important to understand as that means in order to get any benefit from a Signature Dish, the recipe must include seasonings. Simple meals made of a single cooked fish provide no buff and therefore can't benefit from being a Signature Dish.
So, likely the most rewarding of Signature Dishes will be those the the largest buffs, Master Recipes. This was designed to bring yet another level of depth and thought to the cooking system. Also to reward those that take the time to prepare and work at their cooking skills with the concept of using the full breadth of the cooking system.
I hope this clarifies the Signature Dishes concept a bit more for you. Please feel free to post any questions you might have. I will do my best to answer them clearly and completely.