Lay the orange slices in a 6-quart slow cooker. Toss the chicken with the flour, lemon zest, 1/4 teaspoon salt and a few grinds of pepper in a large bowl; add to the slow cooker in a single layer. Combine the chili sauce, soy sauce, ginger and garlic in a bowl; pour over the chicken.
Cover the slow cooker and cook on low, 7 hours. Uncover and gently stir, breaking up the chicken into large pieces; let stand 10 minutes.
Meanwhile, bring a large pot of water to a boil. Add the noodles and cook as the label directs. Drain; toss with the watercress.
Season the chicken with salt and pepper. Serve over the noodles and top with the cilantro.