1.Rinse the chicken breasts under cool water and pat dry with paper towels. Cut the chicken into 2" cubes then set aside.
2.Place a large soup pot over medium heat and add olive oil. When the oil is hot, add the diced chicken and cook 6–8 minutes until cooked through and no pink remains. Remove the chicken from the pot and set aside to cool slightly.
3.While the chicken is cooling, add the onion to the oil remaining in the pot then cook until it becomes translucent—approximately 3–4 minutes. Add garlic and cook for 1 additional minute. Sprinkle flour over the onions and garlic, stir to combine then cook for 1 minute.
4.Add chicken broth, stir to dissolve the flour then add tomato paste, bay leaves and salt. Stir to combine, scraping up any of the bits stuck to the bottom of the pan. Increase the heat, bring soup to a boil then reduce the heat to low.
5.Shred or chop the cooked chicken breast and place it back into the pot. Cover and simmer the soup 30 minutes.
6.Next, stir in the pasta and spinach. Increase the heat to medium-high, bring the soup back to a boil then immediately reduce the heat to medium-low. Simmer uncovered, until the pasta is perfectly cooked.
7.Remove the bay leaves and serve the soup with a pesto-topped crostini at the bottom of your bowl. Sprinkle with plenty of fresh grated Parmesan cheese and enjoy!