I had leftover lasagna one day and turned it into soup doing the following . . . I now make lasagna specifically for making this soup.
Bake your favorite lasagna (the ricotta type . . . and preferably with spinach) . . . once it comes to room temperature, refrigerate until firm. Using a sharp knife, cut lengthwise into 1/2" strips and then cut the in opposite direction in 1/2" strips so that you end up with 1/2" squares (or something close to that). In a large soup pot, add the lasagna and enough homemade (or low-sodium canned) chicken broth to cover by about 4"-5" (you can add more later if it’s too thick) and bring to a simmer and simmer about 30 minutes or until the lasagna breaks down into the broth. Add some finely chopped frozen or fresh spinach if you didn’t add it to your lasagna or leave it out. (Freeze leftovers, if you have any).
Note: I take a baguette and cut it into 1/2" slices and lightly toast them in a toaster. Rub a garlic clove across and add a drizzle of extra-virgin olive oil and then some shredded Italian blend cheese and put under the broiler until golden brown. Serve soup, cheese toasts and a salad with a vinaigrette dressing for a meal.