Heat 2 tablespoons olive oil in a large pot over medium heat for about 1 minute.

Add 1 medium yellow onion, diced, and cook for 4 to 5 minutes, stirring frequently, until translucent and soft.

Stir in 3 cloves garlic, minced, and cook for 1 minute, stirring constantly to prevent burning.

Pour in 6 cups chicken broth and add 2 medium carrots, diced, and 2 celery stalks, diced; bring to a gentle simmer.

Add 1 can (14.5 ounces) diced tomatoes with liquid and 1 can (15 ounces) white beans, drained and rinsed.

Season with 1 teaspoon dried Italian seasoning, 1/2 teaspoon salt, 1/4 teaspoon black pepper, and 1/4 teaspoon red pepper flakes if using; simmer uncovered for 15 minutes.

Stir in 2 cups diced chicken and warm through for 3 to 5 minutes.

Add 2 cups fresh spinach or kale and stir until wilted, about 1 minute, then stir in 1 tablespoon fresh lemon juice.

Taste and adjust seasoning with additional salt, pepper, or lemon juice if needed before serving.