Mercury and her sister ships do not have an alternative extra fee restaurant. The casual dining at the rear of the buffet area is reservation only and the $2/pp is for the waiter's tip. Your entree's are usually chicken, salmon, pasta and maybe one other w/an assortment of salads and appetizers.
You can get the regular dining room food delivered to your room during dining room hours. Talk to your cabin steward if you think you will miss dinner because of a shore excursion, he/she will be able to guide you.