Start the soup: Set a large soup pot or Dutch oven on the stove over medium heat. Add the olive oil to the pot.

Soften the vegetables: Add the chopped onion, sliced fennel, and sliced celery to the pot. Season lightly with salt and black pepper.

Soften the vegetables: Cook the vegetables for 8 minutes, stirring every minute or two, until the onion is soft and the fennel looks slightly translucent.

Build the base: Add the chopped garlic to the pot. Cook for 30 seconds, stirring, until it smells fragrant.

Build the base: Stir the tomato paste and chopped rosemary into the vegetables. Cook for 1 minute, stirring, until the tomato paste darkens a little and coats the vegetables.

Add the broth: Pour the chicken broth into the pot. Stir along the bottom of the pot to loosen any bits stuck there.

Add the broth: Add the drained cannellini beans to the pot and stir them into the broth.

Simmer until tender: Raise the heat to medium-high and bring the soup to a boil.

Simmer until tender: Lower the heat to a gentle simmer. Cook for 12 minutes, until the fennel is tender and no longer crisp.

Warm the chicken: Add the shredded rotisserie chicken to the pot and stir it into the soup.

Warm the chicken: Simmer for 3 to 4 minutes, until the chicken is hot all the way through.

Finish the soup: Squeeze the juice from one lemon half into the pot. Stir, taste the broth, and add more salt and black pepper if needed.