2 teaspoons grated lime zest
2 tablespoons fresh lime juice
2 tablespoons unsalted butter, melted and cooled
1 tablespoon vegetable oil
1 teaspoon sugar
1 cup quinoa
1 (14- to 15-ounce) can black beans, rinsed and drained
2 medium tomatoes, diced
4 scallions, chopped
1/4 cup chopped fresh cilantro
Whisk together lime zest and juice, butter, oil, sugar, 1/2 teaspoon salt, and 1/4teaspoon pepper in a large bowl.
Wash quinoa in 3 changes of cold water in a bowl, draining in a sieve each time.
Cook quinoa in a medium pot of boiling salted water (1 tablespoon salt for 2 quarts water), uncovered, until almost tender, about 10 minutes. Drain in sieve, then set sieve in same pot with 1 inch of simmering water (water should not touch bottom of sieve). Cover quinoa with a folded kitchen towel, then cover sieve with a lid (don't worry if lid doesn't fit tightly) and steam over medium heat until tender, fluffy, and dry, about 10 minutes. Remove pot from heat and remove lid. Let stand, still covered with towel, 5 minutes.
Add quinoa to dressing and toss until dressing is absorbed, then stir in remaining ingredients and salt and pepper to taste.