I cannot remember if spinach is a member of the nightshade family or not off hand; it is cruciferous vegetables that can by thyroid inhibitors (but you would have to eat a lot) and nightshades are generally to be avoided if you have arthritis.
There is no need for it to be cooked, you can get something like Easygreen from Dorwest that contains spinach.
The problem with spinach is it contains oxalates so any dog with certain types of bladder stones should avoid it!
Vitamin C is a natural antihistamine which might help. Desensitisation works on some dogs not all.