Menu is the general word for all the food and drinks your inn offers.
In the Inn Menu window you can add and remove new drinks and dishes. To do so, press the + icon. A window with all recipes available will appear. An exclamation mark next to an item means that you do not have required ingredients.
If you select a recipe you can see its description, properties (energizing, sweet, etc.), price, ingredients (and objects) needed, and which Social Groups likes the given dish.
Once a dish is added to the menu, you can set its price.