Place the chicken breasts in the bottom of a 6-quart slow cooker. (They cook most evenly when they're not stacked.)
Add the beans, diced tomatoes, sun-dried tomatoes, onion, garlic, broth, oregano, basil, red pepper flakes, salt, and pepper. Stir gently to combine.
Cover and cook on LOW for 6 to 7 hours or HIGH for 3 to 4 hours, until the chicken shreds easily with a fork.
Remove the chicken to a cutting board and shred with two forks.
Stir the spinach into the hot bean mixture — it wilts in about 2 minutes.
Return the shredded chicken to the pot and stir. Taste and adjust salt.
Serve over cauliflower rice or whole-grain toast, topped with Parmesan and fresh basil.