Toss cubed tomatillos, garlic (whole), onion chunks, and jalapeno with 1 tbsp oil and place on a baking sheet. Roast vegetables in 450 degree oven for 10-15 minutes, or until slightly charred. Place vegetables in blender with lime juice, cilantro stems, romaine, pumpkin seeds, and salt and blend until smooth. Reserve 1 cup for plating, placing the remaining in a Ziploc bag. Add the chicken pieces to the bag and marinate for at least 1 hour.
Place chicken on sheet pan and place in 450 degree oven for 30-40 minutes, or until internal temperature is 158 degrees. Rest chicken for 5 minutes before serving. Finish with reserved mole sauce.