Place cashews in a bowl and pour boiling water over them and soak while preparing everything else.
In a medium-sized skillet, sauté onions, mushrooms, and garlic in a drizzle of olive oil on medium heat until soft and moisture is cooked out, about 10-12 mins. When finished, set pan aside.
Bring large pot of water to a boil and cook manicotti according to package directions until al dente. Don't overcook!! When al dente, strain manicotti and rinse with cold water. Set aside.
Pre-heat oven to 350°.
Meanwhile, drain tofu and use a clean kitchen towel to pat excess moisture. Use your fingers to crumble tofu into a large bowl. Add in nutritional yeast, salt, finely chopped spinach, lemon juice, and pepper. When mushrooms and onions have finished cooking, add them into the bowl. Mix this together thoroughly.
Stuff the pasta with the manicotti filling, however it's easiest for you. I hold the pasta upright and use my pinky to prevent filling from falling out while I spoon it into the top. Place the stuffed pasta in a large casserole dish (or two smaller ones). If you have leftover filling, spoon that in front of and behind the pasta.
Pour the pasta sauce on top of the manicotti. Cover the casserole dish a lid (or foil if you don't have a lid) and bake for 20 minutes. Remove lid (or foil) and bake for an additional 10 minutes.
While manicotti bakes, make the melted cashew cheese. Strain water from cashews and add to a high speed blender along with everything else in the "Melty Cashew Cheese" section. Blend until smooth and creamy.
When manicotti finishes cooking, drizzle the cashew cheese on top of the marinara. Serve immediately, garnishing with optional chopped basil, chives, or green onions. Enjoy!