While the sausage cooks, prepare all your vegetables. In a large pot or Dutch oven over medium-high heat, break apart the Italian sausage and cook until browned and crumbled, about 5-7 minutes. While the sausage is cooking, dice the onion into 1/2-inch pieces, chop the red pepper into 1/2-inch chunks, dice the celery, slice the carrots into 1/4-inch rounds, and mince the garlic cloves. Once the sausage is cooked through, carefully drain off excess fat, leaving about 1 tablespoon in the pot for flavor.
Add the diced onion, red pepper, celery, and carrot slices to the browned sausage and sauté over medium heat for 3-4 minutes, stirring occasionally. This allows the vegetables to soften slightly and begin releasing their natural sugars. Add the minced garlic along with the Italian seasoning, salt, pepper, and bay leaf, then cook for another 1 minute until fragrant. I like to add the garlic last because it's delicate and cooking it too long can make it bitter—just that one minute is enough to bloom the flavors.
Pour in the chicken broth, water, and fire roasted Italian tomatoes (with their juice) directly into the pot. Stir well to combine everything and bring the mixture to a gentle boil over medium-high heat. Once boiling, reduce the heat to medium and let it simmer for about 5 minutes so the flavors meld together. This step builds the foundation of your soup before adding the pasta.
Add the ditalini or small shell pasta directly to the simmering broth and stir well to prevent sticking. Cook for about 10 minutes, stirring occasionally, until the pasta is tender but still holds its shape. The pasta will absorb some of the broth and thicken the soup slightly, which is exactly what you want.
Stir in the fresh spinach and cook for 1-2 minutes until it wilts completely into the soup. Taste the soup and adjust the seasoning with additional salt and pepper if needed—I always taste before serving because the fire roasted tomatoes and sausage already bring a lot of flavor. Remove the bay leaf before serving. For a final touch, I like to finish each bowl with a small grating of Parmesan cheese and fresh basil or parsley.