From America's Test Kitchen
Mature supermarket green beans are often tough and dull, needing special treatment to become tender and flavorful. Braising works, but the stovetop can get awfully crowded as dinnertime approaches. Roasting is a great option for many vegetables, and we wanted to find out if this technique could help transform older green beans, giving them a flavor comparable to sweet, fresh-picked beans.
A remarkably simple test produced outstanding results: Beans roasted in a 450-degree oven with only oil, salt, and pepper transformed aged specimens into deeply caramelized, full-flavored beans. Just 20 minutes of roasting reversed the aging process (converting starch back to sugar) and encouraged flavorful browning. Just 1 tablespoon of oil was enough to lend flavor and moisture without making the beans greasy. Lining the pan with foil prevented scorching and made for easy cleanup.
An aluminum foil liner prevents burning on dark nonstick baking sheets. When using baking sheets with a light finish, foil is not required, but we recommend it for easy cleanup.