Heat the olive oil in a large Dutch oven or soup pot over medium-high heat. Add the Italian sausage and cook for 5–7 minutes, breaking it into small pieces, until thoroughly browned and no longer pink.

Add the diced onion to the browned sausage and cook for 3–4 minutes, stirring occasionally, until the onion softens. Scrape the bottom of the pot as you stir to loosen flavorful browned bits.

Stir in the minced garlic, Italian seasoning, and optional crushed red pepper flakes. Cook for about 30 seconds, just until the garlic is fragrant.

Pour in the chicken broth, then add the drained beans, diced tomatoes with their juices, kosher salt, and black pepper. Stir well and bring the soup to a boil.

Reduce the heat and simmer the soup uncovered for 15–20 minutes. This gives the sausage, beans, tomatoes, and broth time to blend into a rich, savory flavor.

Stir in the baby spinach during the final 1–2 minutes of cooking. Continue stirring just until the spinach wilts while retaining its fresh color.

Taste the soup and adjust the salt and pepper as needed. Ladle into bowls and serve with grated Parmesan and crusty bread, if desired.