Add the onion, brown mushrooms, zucchini, garlic, tomato sauce, crushed tomatoes, vegetable broth, bay leaves, oregano, basil, red pepper flakes, kosher salt and black pepper to the liner of a 6 quart slow cooker.
Set the slow cooker on low and cook for 7 hours or on high and cook for 3 1/2 to 4 hours.
Once the cooking time is done, cook the lasagna noodles according to the package directions, drain and rinse and add to the soup.
Cut them into smaller pieces. Add the spinach and warm until wilted then ladle into soup bowls.