Pick over and rinse the lentils and put them in a 4-quart saucepan. Cover them with 2 quarts cold water and add 2 teaspoon salt. Bring to a boil over high heat. Lower the heat, partially cover the pot, and simmer until the lentils are slightly firm but tender and still intact, 20 to 30 minutes. Drain and run cold water over them. Drain again and transfer to a large bowl.

Sprinkle the lentils with the vinegar, 1 teaspoon salt, and a few grinds of black pepper. Stir gently to combine, taking care not to mash the lentils. Stir in the oil, 1 tablespoon at a time, and finally, the scallions and garlic.

Let stand at room temperature for about 1 hour; then season to taste and serve.

For the best flavor, make this salad at least 1 hour and up to 3 hours before serving.