Step 1: Heat a large, heavy-bottomed pot or Dutch oven over medium-high heat. Add 1 tablespoon of olive oil, then crumble the Italian sausage into the pot, breaking it up with a spoon. Cook until thoroughly browned and no longer pink, about 6-8 minutes. Remove the browned sausage with a slotted spoon and set aside, leaving the rendered fat in the pot.
Step 2: Reduce the heat to medium. Add the chopped onion to the pot and cook, stirring occasionally, until softened and translucent, about 5-7 minutes. If needed, add a splash more olive oil.
Step 3: Stir in the minced garlic and dried Italian seasoning. Cook for another minute until fragrant, being careful not to burn the garlic.
Step 4: Pour in the crushed tomatoes and chicken broth, scraping up any browned bits from the bottom of the pot with a wooden spoon. Bring the mixture to a simmer.
Step 5: Add the rinsed cannellini beans, return the browned sausage to the pot, and stir everything together. Season with salt, black pepper, and a pinch of red pepper flakes (if using) to taste.
Step 6: Bring the stew back to a gentle simmer, then reduce the heat to low, cover, and cook for at least 15-20 minutes to allow the flavors to meld. For a deeper flavor, you can let it simmer longer, up to 30 minutes, stirring occasionally.
Step 7: Just before serving, stir in the fresh spinach (if using) until wilted, about 2-3 minutes. Taste and adjust seasonings as needed. If the stew is too thick, add a little more broth until it reaches your desired consistency. Serve hot.