A little gem we discovered on our most recent trip is Da Carletto. I have been trying to find out whether they are open on Sundays, but have been unsuccessful. Perhaps, your concierge would know.
The food is fresh and the ambiance is so inviting. This is a tiny restaurant, only about 10 tables. A Venetian friend recommended Da Carletto and we returned about 3 times during a 10 day visit. It is very close to the Santa Maria Formosa Church. The hotel where we stay is nearby and when we got lost, the daughter of the owner came and led us to the restaurant, which was about a minute away. We made a wrong turn at the church. This is a true family-owned restaurant. Mom is in the kitchen and daughter is serving the tables. I recall having a perfectly simple grilled fish (likely, Branzino) and grilled vegetables. Chef/Mom makes an extraordinary Apple cake. The preparation of Spider Crab is outstanding. I believe I had it with pasta, but I would not be surprised if they modified the dish for you, without pasta.
One reliable Sunday restaurant, which does not get a lot of love from critics, is the legendary Harry's Bar. Harry's is a lot of fun on Sundays, particularly at lunch. It is filled with locals who have been going there forever on Sundays.
Our experiences at Harry's have been fine, and we know how to order so that we won't break the bank. They are very good about splitting orders and even letting you order a 1/2 portion. The Bellini's are outrageously expensive but a carafe of prosecco is reasonably priced. Service is excellent. Sunday is the day they prepare Osso Bucco and it is well worth trying. I rarely eat meat, but this Osso Bucco is phenomenal. Lunch is our preferred dining time at Harry's on Sunday. Gives us time to have a leisurely lunch and go for a good walk after. Reservations definitely needed on sundays. Request a table downstairs. It is where the locals dine, although Elton John prefers dining upstairs.