Blend ancho chiles, pasilla chiles, cumin, onion, salt, and pepper together using a spice or coffee grinder into a fine powder. Pour spice blend into a bowl and stir in brown sugar.

Squeeze lime juice over turkey and rub with spice blend. Let marinate for about 1 hour.

Preheat an outdoor grill for high heat and lightly oil the grate.

Reduce grill temperature to medium. Remove turkey from marinade, reserve any excess for basting. Sear turkey breast on the preheated grill, 3 to 4 minutes per side.

Turn 1/2 of the grill burners off. Roast breast on the grill for 15 to 20 minutes. Turn turkey over and baste with reserved marinade; cook until an instant-read thermometer inserted into the center reads at least 165 degrees F (74 degrees C), 15 to 20 minutes more. Let sit for 5 minutes before serving.