Drain and reserve 1 cup pasta cooking water. Heat a large skillet over medium-high heat. Add olive oil, garlic and bok choy, and sauté until greens wilt and release their liquid, 4–5 minutes.

Cook spaghetti according to package directions; drain. Meanwhile, trim and discard root end of bok choy. Cut stalks into 1-inch pieces. Coarsely chop leaves. In a large skillet, heat 1 tablespoon ...