The acid in tomatoes can dull the color of green beans in unappetizing ways. We kept the colors and flavors bright and fresh by adding chopped grape tomatoes only at the very end of cooking. We liked small or baby Yukon Gold potatoes because they are waxier and hold their shape better than larger potatoes; if small Yukons aren't available, use small red potatoes. We preferred the flavor and texture of brown mustard seeds, but if you have difficulty finding them, yellow mustard seeds will work. Make sure your skillet has a tight-fitting lid.