Heat a medium, heavy skillet (not nonstick) over high heat for 2 minutes.
Coat pan with cooking spray. Immediately add green beans to pan, shaking them into a single layer; cook, without stirring, 2 minutes or until beans are very lightly charred. Cook beans 5 more minutes or until crisp-tender and evenly charred, tossing occasionally.
Remove pan from heat. Let beans rest 1 minute. Add butter; toss until butter melts and coats beans. Add vinegar; toss. Turn on heat if necessary to evaporate most of liquid. Sprinkle beans with tarragon and salt; toss. Serve immediately.
Pan-Charred Asparagus: In place of green beans, use 8 ounces (2 1/2-inch) pieces of trimmed asparagus. Use 1 1/2 teaspoons toasted walnut oil in place of butter and 2 teaspoons lemon juice in place of vinegar. After tossing asparagus with tarragon and salt, sprinkle with 1 tablespoon chopped, toasted walnuts and 2 tablespoons shaved Parmigiano-Reggiano.
Serves 4 (serving size: about 2/3 cup)
Calories 52; Fat 4 g (sat 0.7g); Sodium 160mg