Valencia Restaurant for Christmas
I haven't been to Valencia yet but I'm pretty good at doing restaurant research on the Net so, hope this helps.
Spain doesn't observe Boxing Day. While you wait for specific recommendations, you might do some browsing on atrapalo.com. Although you'd have to register yourself to use the reservations service (input a dummy Spanish postal code), at least you can see what days the restaurants are open. For example, the link below is for Valencia's La Succursal, a 1-star Michelin in the IVAM (modern art museum). According to the reservations calendar, it's closed on X'mas day but open Dec. 26.
On my personal list of where to eat in Valencia, I have Ca Sento, La Cuina de Boro and Lloc de Tapas.