Preheat the oven to 400°F.
Cook the jumbo shells according to the package directions. Once cooled enough to touch, separate them so they won't stick together on a plate. Set aside.
In another pot, boil the cashews for 15 minutes.
While the shells and cashews are boiling, heat the oil in a pan over medium heat. When hot, add the mushrooms and cook until browned (about 4 minutes).
Add the spinach and cook, covered, until wilted. Set aside.
On a plate, use a fork to smash the tofu completely. It will be completely crumbled when done. Transfer to a bowl.
Add the cashews and the ½ cup water into a blender and blend until smooth. Add the lemon juice, nutritional yeast, garlic powder, and yellow miso paste (if using) and blend until combined. Add the salt and pepper to taste.
Add the cashew sauce, mushrooms, and spinach to the bowl with the tofu and combine. Mix thoroughly and add more salt and pepper, to taste.
In a baking pan or pie pan, spread a thin layer of marinara sauce along the bottoms of the pan.
Spread a layer of marinara sauce inside one of the shells, then fill the shell with the ricotta mixture. Place in the pan and repeat until the pan is filled with shells (or you run out of shells).
Spread additional sauce on the outside of the shells that are visible and in the spaces in between the shells. The gaps do not need to be filled with sauce, but you should try to make sure that exposed shells have some sauce on them.
Place the pan in the oven and bake for 15 minutes. Remove and let cook for approximately 10 minutes before serving.