To prepare marinate: In a large bowl, use a whisk to combine cilantro, garlic, juice and broth. Set aside 1/4 cup of marinade and pour the rest into a shallow baking dish. Arrange the chicken into the dish, cover and place in the refrigerator to marinate - about 20 minutes.

To cook chicken: Remove chicken from the marinade, pat-dry with paper towel and season liberally w/ salt & pepper. Place a large overproof skillet on the stove over a moderate high heat and add oil. When the oil is simmering, add the chicken and cook until golden brown on both sides, about 3 minutes per side. Transfer the chicken to the over (400F) and cook until cooked through about 8 minutes. Using tongs, remove the chicken from the over and transfer to a place and cover with foil

Return skillet to stove over a medium high hear add the rest of the marinade and zest, simmer until thick about 3 minutes. remove from stove and toss in spinach.