In the slow cooker, combine the crushed tomatoes, vegetable or chicken broth, diced onion, minced garlic, Italian seasoning, sugar, salt, and black pepper.
Cover and cook on low for 4 hours.
After 4 hours, stir in the cheese tortellini and spinach. Cover and cook on low for an additional 30 minutes until tortellini is cooked and spinach is wilted.
While the soup is cooking, preheat the oven to 375°F (190°C) for the cheddar toast bites.
Spread the softened butter on one side of each slice of bread. Place the slices on a baking sheet butter-side up.
Sprinkle shredded cheddar cheese generously on top of each slice of bread.
Bake in the preheated oven for about 10-12 minutes, or until the cheese is bubbly and the bread is toasted.
Serve the soup hot in bowls, garnished with any remaining shredded cheese if desired, and enjoy with the cheddar toast bites.