The restaurant specializing in mushrooms is called La Truffe
The other restaurant -veggie/healthy/organic- is I think
Entre Ciel et Terre. I am not
sure if they are low-fat if you don't specially check(they
have rich desserts). However, they have good food,
interesting flavors/textures -vegetables and grains. They
had only about 12 tables so call for reservations-we went on
a Friday night in February and got the last table.
Right near Notre Dame is Grenier du Notre Dame which (if I
remember) does more tofu/seitan type stuff along with veggies.