Will be in Marseille on a Sunday in May and looking for a nice, non-touristy restaurant around Old Town for lunch.
Many of the restaurants that I have seen on internet are closed on Sundays.
Enjoy local French cusine; , duck, salads, cheese, etc.
Outside dinning would be a plus.
I'll assume that by "Old Town" you mean the Panier area?
Restaurants there and along the Vieux Port are hit-or-miss. I've had good, relatively recent experiences at Au Bout du Quai, right on the Vieux Port, close to Mucem. They have a terrace and are open Sunday.
Within the Panier itself, most places with a terrace don't look great. Au Coeur du Panier has been recommended to me, but I never tried it.
Duck is not local to Marseille, by the way - it's a southwestern dish, not Provençal.
Marseille —> Bouillabaisse .
The Chez Madie there is the original one that inspires the many many that take on that name in other towns in France - it is touristy but with French tourists:
https://restaurantchezmadielesgalinettes.eatbu.com/?lang=fr
Because it is so renowned the standard all around it is pretty high so if you can't get in there pick one of the others nearby.
Thanks for the recommendations
We visited on a Sunday and had a very good lunch at Restaurant Femina