Caldo de res recipe

Bone-in beef shanks (about 2 pounds) Vegetable oil (1 tablespoon) Salt (2 teaspoons) Ground black pepper (2 teaspoons) Onion (1, chopped) Beef broth (3 cups) Diced tomatoes (1 can, 14.5 ounces) Water (4 cups) Carrots (2 medium, coarsely chopped) Fresh cilantro (¼ cup, chopped) Chayotes (2, quartered) - optional Corn (2 ears, husked and cut into thirds) Potato (1, quartered) - optional Cabbage (1 medium head, cored and cut into wedges