I planted mustard green seeds about three years ago and promptly forgot about them. Since then, every fall and spring when it's cool out and nothing too exciting's around for harvest, the rich canvas of their dark leaves catches my eye amidst an otherwise brown plantscape, and I'm grateful to head inside with a handful of their leaves to spunk up a salad, brighten a burrito, or spice up a soup or stew.
Mustard greens are fantastic in a salad mix, perfect in spicy recipes, or cooked down they mellow out and do just as well as spinach in any dish. Once cooked, their "bite" diminishes profoundly, much like arugula.
These cholesterol-lowering, cancer-protective, gorgeous, long-lasting, low to no-maintenance veggies are also sky-high in vitamin C, A, and K. This is one mildly "invasive" garden species I can definitely live with.