Use a tofu press to press any excess water from the tofu. Alternately, wrap the tofu in several layers of paper towel, place it on a rimmed cutting board or large plate, and set a heavy cast iron skillet on top of it. If you don't have a cast iron skillet, set a pan on top of it and place several heavy books or cans in the pan. Press the tofu for 15 to 20 minutes.
While the jumbo shells cook and the tofu presses, preheat the oven to 400F, lightly grease a casserole dish, and get started on the filling.
Heat the olive oil in a large saute pan over medium heat. Add the chard, kale, and garlic, and cook for 3 to 5 minutes, or until the greens wilt and soften. Remove from the heat and let cool slightly. If you prefer smaller pieces of greens (I do), turn the mix out onto a cutting board and chop to desired size.
Add the pressed tofu to a food processor and pulse several times until the mixture resembles ricotta cheese. Transfer to a large mixing bowl and stir in the greens, herb/s, nutritional yeast, lemon juice, sea salt, black pepper, and red pepper flakes (if using).
Taste and adjust seasonings, adding more salt and/or pepper (I usually add another 1/4 teaspoon sea salt).
Spoon 1 to 1 1/2 cups of the marinara into the bottom of the casserole dish, Use the back of a spoon to evenly spread.
Stuff each shell with about 3 tablespoons of the tofu ricotta mixture, and transfer to the casserole dish. You should have about 16 generously stuffed shells. Spoon a bit of marinara over each shell, reserving some sauce for serving.
Cover the dish with foil, poking several slits across the surface to vent.
Bake for 20 to 25 minutes, or until warmed through.
Top with additional marinara, if desired, and serve immediately.
Cover and refrigerate leftovers.