In a 3 ½ to 4-quart slow cooker, combine the undrained diced tomatoes and condensed cream of mushroom soup.
Stir until well-mixed.
Add the sliced carrots, celery, onions, minced garlic, Italian seasoning, salt, and pepper, and stir to combine everything thoroughly.
Cover the slow cooker and cook on the low-heat setting for 7 to 8 hours or on the high-heat setting for 3 ½ to 4 hours, until the vegetables are tender.
Just before the sauce is finished, cook the multigrain spaghetti according to the package directions.
Drain well.
Gently stir the cubed tofu into the hot, drained spaghetti.
To serve, divide the spaghetti and tofu mixture among plates and generously spoon the hot tomato and vegetable sauce over the top.
Sprinkle with shredded cheddar cheese and enjoy immediately.