I've lived here for many years and El Monasterio has always been the place to go for special occasions. We've never had a bad meal there and it's always been an enjoyable outing. Friends of mine book their New Year's Eve months ahead in one of their dining rooms.
We sometimes go there in the afternoon for coffee and cake and I've been with friends for lunch. There are several restaurants in this old monastery complex and you can surely find something to your liking, be it outside dining on the patios or in one of their dining rooms.
I've made a photo album of one of my visits there with one of my English friends who likes to go for outings with me.
Here's the link to my photo album:
http://www.pbase.com/canarybird/elmonasterio&page=all I'd suggest you go for a visit there during the day and see how you like it. It's very pretty and you have a great view over the Orotava Vallery.
Sharon