Heat oven to 400° F. Heat 1 tablespoon of the oil in a large skillet over medium-high heat.
Season the chicken with the cumin, cayenne, and ½ teaspoon salt. Cook until browned, 3 to 4 minutes per side.
Transfer to a baking sheet and roast until cooked through, 6 to 7 minutes.
Return the skillet to medium heat and add remaining tablespoon of oil. Add the onion and bell pepper and cook, stirring, until soft, 6 to 8 minutes.
Meanwhile, thinly slice the collard leaves crosswise. Add to the skillet along with ½ teaspoon salt and ¼ teaspoon pepper.
Cook, tossing, until just tender, 2 to 3 minutes. Serve with the chicken.