My boyfriend (vegetarian) and I (not) were in Paris during the first week of May and we were staying in a small B&B in the 5th on Rue des Grands Degres. We weren't sure if we were going to have a difficult time finding places with vegetarian options in Paris (we were occasionally stymied in certain areas but for the most part it wasn't a big problem), but we were thrilled when, by chance, we came across Le Grenier de Notre-Dame, at 18 rue de la Bûcherie (basically just south of Notre Dame Cathedral on the Left Bank).
It looks like a tiny place and when you walk in you only see about 6 tables, but upstairs they have a lot more seating. We happened by on Wednesday night (May 2) and decided to try it out as they had tables available, probably around 7:30pm. The menu is basically vegetarian and there were probably some vegan options, but we weren't really looking for that, though I notice now on their website (www.legrenierdenotredame.net) that at least one of their main dishes has fish.
We started with some healthy blender drinks, which I don't see on the online menu. I got something with ginger and carrot (I think), which turned out to be too gingery for me to enjoy, while he got something with celery and tomato, which he loved.
For the main dish, I decided to be adventurous (for someone who isn't very familiar with vegetarian protein options) and got the couscous with seitan (wheat gluten?) brochette, which was pretty good. There was a ton of food on the plate and I definitely couldn't finish everything. He got the cassoulet and was very happy with it. We were very full and decided not to get dessert.
My boyfriend was so pleased with the food, and the healthy blended drink (which he said was so good for him that it probably added seven years onto his lifespan), that he wanted to eat there again, so we returned on Saturday night. We hadn't made a reservation and there wasn't a table available when we arrived, but they said one would open in about five minutes, so we waited. Again we ate downstairs in the smaller area. We started with the blended drinks again but this time I ordered the banana, strawberry, and orange and it was delicious, while he got the same one as before. For dinner, I went the "familiar" route with the fried pasta with cheese and garlic and enjoyed it quite a bit. He ordered the Protein Special and we were both astounded by the amount of food he received. I don't think he finished it but made a valiant attempt! As far as I remember, he thought it was very good. We decided to go for dessert this time (as it was our last night in Paris) and ordered mousses: he got the one with caramel (not on the online menu) and had the basic chocolate. We agreed that they were fairly ordinary. I recommend finding dessert elsewhere in the area.