Paris Restaurants Open on Sunday
Take a look at Chez Janou. It's really fabulous, fairly moderate, and open every night. The portions are extremely filling, and I'd recommend not getting a full-course meal, which also reduces the price. Wonderful wine selection, and a joyful atmosphere. They're in the 3eme.
For years,on Sunday, we have eaten at the Terminus Nord outside of the train station Gare du Nord before catching our train home. The restuarant is reasonably priced, full of Parisians and specializes in fish. Parisian friends took us there in the 1970's and we have had our "last meal" in Paris there ever sense.