Preheat oven to 400°F
Line two baking trays with clean tea towels (or paper towel). Arrange eggplant and zucchini slices in a single layer and sprinkle with salt. Allow vegetables to release moisture (about 1 hour) and dab dry.
Meanwhile, in a large saucepan, combine tomato sauce and 1 cup of water and bring to a boil. Add the lentils and bring to a simmer over low heat. Cover loosely and stir occasionally. Cook for 15 minutes until lentils are tender (this step can be done a day in advance and stored in the fridge).
In a casserole dish, spread a thin layer of lentil tomato sauce, a single layer of eggplant noodles, another layer of lentil tomato sauce, and a sprinkle of ricotta; repeat with zucchini noodles, and then eggplant or/and zucchini noodles (depending on how many noodles you have left of each). Top the last layer with both ricotta and mozzarella.
Bake lasagna for 40 minutes and allow to rest for 10 minutes before slicing.