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Adjust rack to top position (middle and top positions for 4 servings) and preheat oven to 425 degrees. Wash and dry all produce.
Dice potatoes into ½-inch pieces. (TIP: For a smoother texture, peel potatoes first.) Strip thyme leaves from stems until you have 1 tsp (2 tsp for 4); mince leaves. Reserve remaining thyme sprigs for the next step. Trim and thinly slice scallions, separating whites from greens; mince whites. Trim green beans if necessary. Halve, peel, and thinly slice onion. Peel and mince garlic.