Sort 1 pound (about 2 cups) lentils and discard debris. Rinse and drain lentils.
In a 3- to 4-quart pan, combine lentils, 6 cups fat-skimmed chicken broth, 1/2 teaspoon cumin seed, 2 tablespoons minced fresh ginger, 3 tablespoons mustard seed, 1/4 cup rice vinegar, and 1/2 lemon, thinly sliced and seeded.
Bring to a boil over high heat. Cover and simmer until lentils are tender to bite about 45 minutes. Drain, reserving liquid. Let both mixtures cool.
Add to lentils 1 cup chopped green onions (including tops), about 1/2 cup reserved liquid (enough to moisten salad), and salt to taste.