Start by melting 1 tablespoon of butter in your large stockpot over medium heat. Once melted, add the chopped onion, carrots, and celery. Sauté these until they are tender and fragrant, about 5-7 minutes.
Sprinkle in 1 teaspoon of Italian Herb Seasoning, ½ teaspoon each of ground sage and thyme, plus salt and freshly ground black pepper to taste. Stir to coat the vegetables evenly with the herbs, letting the aromas develop for about 1 minute.
Pour in 8 cups of chicken broth along with 3 tablespoons of chicken soup base. Stir to combine. Gently place the 4 large boneless, skinless chicken breasts into the pot. Bring the soup to a boil, then reduce heat to low and let it simmer until the chicken is cooked through, roughly 20-25 minutes.
Remove the chicken breasts from the pot and shred them with two forks or chop finely. Return the shredded chicken to the pot.
Add the frozen cauliflower rice and stir well. This will give the soup a satisfying texture and soak up the flavors nicely. Next, mix in ¼ cup of basil pesto, allowing it to melt into the broth for a creamy, herbaceous finish.
Finally, stir in 2 tablespoons of fresh-squeezed lemon juice. This brightens the soup and balances the richness of the pesto and chicken broth. Taste and adjust seasoning if needed.