If you don't have stone-ground mustard already in your fridge, Dijon would work equally well in this bright and tangy veggie side dish. You can also opt to use microwave-in-bag green beans instead of the slightly more delicate haricot verts--just increase your cook time in the pan by 1 to 2 minutes, or until beans are crisp-tender.
Cook beans according to package directions. Combine vinegar, mustard, salt, and pepper in a bowl. Heat oil in a large skillet over medium-high heat. Add beans; sauté 3 to 4 minutes. Add mustard mixture; toss.