Cut cauliflower into florets and slice them into 1/2-inch-thick slices (you should have about 6 cups). Place cauliflower, tomatoes, beans, corn and cilantro in a large bowl. Sprinkle with chili powder and cumin and stir to combine.
Coat inside of oval slow cooker bowl with nonstick cooking spray. Spread a scant 3 cups cauliflower mixture over bottom of slow cooker, then sprinkle with 1 cup Monterey Jack cheese and a generous 1/2 cup salsa over top. Place 2 tortillas on top. Repeat layering two more times, setting aside last 2 tortillas. Cut these tortillas into 2-inch pieces and scatter over top.
Cover and cook on HIGH for 3 hours or LOW for 5 1/2 hours or until cauliflower is tender. Top with remaining 1/2 cup cheese. Cover and cook another 30 minutes or until cheese has melted. Let sit for 10 minutes, then serve with sour cream, if desired.