Place the chicken breasts in the bottom of your crockpot.
Pour the salsa over the chicken, then sprinkle in the cumin, smoked paprika, garlic powder, onion powder, salt, and pepper.
Add the chopped bell peppers on top.
Cover and cook on low for 4–5 hours or high for 2–3 hours, until the chicken is tender.
Shred the chicken directly in the crockpot using two forks.
Stir in the spinach, if using, and let it wilt for 2–3 minutes.
Taste and adjust seasonings as needed. Serve warm with your choice of toppings.