Similar to broccoli raab, choy sum (also spelled choi sum) has thick stalks and large flower buds. Leaves look a bit like spinach but with pointed tips. Choy sum has a mild mustard or spinach flavor and can be used as you would use mustard greens. It pairs well with strong flavors like shallots, garlic and chile peppers. It is available year-round with peak supplies from late fall through spring.