Not a restaurant, but very much worth mentioning. Can Luc is a marvelous cheese shop in Gracia, just next to Mercat de la Llibertat. They have a huge selection of cheeses, many of them amazing French ones. The owner is very knowledgeable and can help you with anything cheese-related.
They have lots of other very interesting food items for sale that you can’t find anywhere else. I’ve bought an amazing smoked salt, a truffle salt, and orange blossom water. And according to the owner they have the best selection of champagnes in the city..!