I can't remember ever ordering a gratinée myself, so I don't know where to take my friend with the memory (?) of wonderful onion soup here 20 years ago. These days it appears to be offered at places that probably use canned soup and an excessive amount of cheese on top to sell to tourists.
He doesn't want, and cannot afford, a fancy restaurant. Perhaps a café or even a take-out. I was thinking of the marché des Enfants Rouges, with its many food booths, but don't know if anyone sells onion soup there. My apt is near the rue St. Paul, so the closer the better to there.