There are so many restaurants in Venice that it would be impossible to recommend just one. Someone may be able to tell you of one but you would then have to find out exactly where it is and that is not very easy in Venice.
Your best bet would be to spend some time browsing the menus at the better looking restaurants as you wander round and then keep a shortlist of the ones you like the look of.
BTW. Just because there are many expensive restaurants around St Marks it does not mean they are the best. Try looking in the smaller squares slightly away from the main tourist traps of St Marks and Rialto.
I like the Canareggio area for food but that might be a bit too far to walk.