Cook the lentils: In a medium saucepan, combine the lentils and water and bring to a boil. Cover, reduce the heat, and simmer, stirring occasionally, for 17 to 20 minutes or until tender but not mushy. Drain any excess water, let cool. Use in any recipe that calls for cooked lentils.
Make the Lemon-Herb Dressing: Transfer the cooked lentils to a medium bowl. Stir in the lemon juice, olive oil, salt, mustard, and pepper. Stir in the parsley and red pepper flakes, if using. Serve as a side dish or store in the refrigerator for up to 5 days.