The one restaurant a friend recommended we go to in Montreal was Jardin Nelson simply for their French onion soup. This place is in Old Montreal and the inside doesn't let you forget that with the stone walls. It's cool!
All we ordered was the French onion soup and it was amazing! One bowl was enough for each of us and we slowly ate it to savor every bite. It also helped that it was pouring down rain so soup was the perfect lunch!
Definitely stop here to try heir French onion soup if you're visiting Old Montreal.