Sunday is the night Le Comptoir du Relais doesn't do its much-sought-after gourmet dinner, but rather offers an a la carte menu; probably not coincidentally, it's the one night you can get a reservation without too much difficulty. The food's good, by no means great; I'd rate it slightly better than l'Ardoise.
Both Joel Robuchon restaurants (l'Atelier and la Table) are open on Sunday night. La Table's more comfortable (and takes reservations for the entire evening); I recall the menu being similar to l'Atelier's, but perhaps a bit more limited, and slightly pricier.