Prepare the marinara sauce (below) and cook the lasagna noodles according to the package directions. In a small bowl, combine the ground flax with hot water. Whisk and let stand about 5-10 minutes. Preheat oven to 350°F.
Using a large saucepan, sauté the garlic in the olive oil for about one minute. Add spinach and cook until wilted (about 5 minutes). Add the chopped mushrooms, salt, pepper, and sage. Continue to cook another 5-6 minutes, until the mushrooms are tender.
In a large bowl, combine the spinach and mushrooms with the flax egg, cottage cheese, and Parmesan. Adjust salt and pepper as needed.
Scoop about one cup of marinara sauce in the bottom of a 9×13 pan and spread it around. Lay the lasagna noodles on a large cutting board or piece of freezer paper. Scoop about 1/3 cup of the filling onto each noodle and spread out. Roll each noodle and place in the pan (seam side down).
Scoop another cup of marinara sauce over the rolls and top each one with a slice of fresh mozzarella cheese.
Cover the pan with aluminum foil and bake for 40 minutes or until the cheese is melted. Let cool for a few minutes before topping with extra sauce and serving.