Preheat the oven to 375ºF.
Place the tofu on a paper towel-lined plate. With another paper towel or clean dish towel, press the tofu to remove some of the water.
Crumble the tofu into a food processor and pulse together until crumbly. Add the remaining ricotta ingredients (minus the spinach) and pulse until mostly smooth. Pulse in spinach.
Transfer the ricotta to a piping bag (or make your own with a plastic storage bag - watch the video above for instructions!), and set aside.
For the sauce, mix together the tomatoes, spices and salt. Set aside.
Cook the manicotti according to package instructions. Rinse under cold water and set aside until they're cool enough to handle.
Once ready to assemble the dish, add 1/2 the tomato sauce onto the bottom of a 9x13 baking dish. Using the piping bag, fill the manicotti shells with tofu ricotta. I found it easiest to do one half and then fill the other half. Place the filled pasta into the dish and continue until all the noodles are filled.
Sprinkle the top with cheese. Cover the dish with aluminum foil and bake on the center rack for 30 - 35 minutes. With 5 minutes left on the baking time, remove the foil.
Allow the manicotti to cool for at least 5 minutes, then transfer to plates and sprinkle with red pepper flakes.