To prepare the adobo, heat 1 tablespoon oil in a large nonstick skillet over medium-high heat. Add anchos; sauté 2 minutes or until anchos are blistered. Combine anchos and 2 cups water in a small bowl. Cover and let stand 20 minutes or until tender. Combine ancho mixture, 1/4 cup vinegar, and next 7 ingredients (1/4 cup vinegar through garlic) in a blender; process until smooth. Strain marinade through a medium-mesh sieve into a bowl; discard solids. (Adobo will have the consistency of steak sauce.)
To prepare remaining ingredients, remove and discard giblets, neck, and skin from chicken. Rinse chicken with cold water; pat dry. Trim excess fat. Place the chicken, breast side down, on a cutting surface. Cut in half lengthwise along the backbone. Combine 1 cup marinade and chicken in a large zip-top plastic bag; seal and marinate in refrigerator 4 hours.
Preheat oven to 375°.
Remove chicken from bag, reserving marinade. Place chicken on a broiler pan or rack of roasting pan. Insert meat thermometer into meaty part of thigh, making sure not to touch the bone. Pour 1/3 cup water into pan. Combine potatoes, 1 tablespoon oil, and 1/4 teaspoon salt in a small bowl. Arrange potatoes around chicken. Bake at 375° for 1 hour or until thermometer registers 160°. Brush the reserved marinade over chicken and potatoes, and bake an additional 10 minutes. Place chicken and potatoes on a platter. Combine onion and 1 tablespoon vinegar in a bowl; arrange around chicken and potatoes. Garnish with watercress sprigs, if desired.