If using frozen edamame, let thaw while you chop up the other vegetables. Chop up vegetables into bite sized pieces, and combine in big bowl. For the Dressing: The dressing will make about twice as much as you'll need for the Succotash, but it's good, so nice to have in the fridge for a salad, or for next time you make this dish! Put your mustard, garlic and lemon juice in a small bowl (or pitcher) and combine well with a fork. Add the olive oil, pouring slowly as you combine. Whisk well with a fork –the mixture will be quite thick. Taste and adjust ingredients to your liking. Season with salt and pepper. Feel free to add a little mild vinegar if you want more of a kick. Pour about half of the dressing on top of the chopped vegetables, and mix. Store the rest in the fridge. Dressing stores in fridge for months.