How to Make It
Heat oven to 400° F. Bring 4 cups water to a boil.
Add the lentils and 1 teaspoon salt and simmer, stirring occasionally, until tender, 20 to 30 minutes. Drain and rinse under cold water to cool.
Meanwhile, in a small bowl, combine the balsamic vinegar and brown sugar.
Heat 1 tablespoon of the oil in a large ovenproof skillet over medium-high heat. Season the pork with ¼ teaspoon each salt and pepper. Cook, turning occasionally, until browned, 6 to 8 minutes.
Transfer the pork to oven. Roast until cooked through, 10 to 12 minutes, basting with the glaze twice during the last 5 minutes. Let rest before slicing.
In a medium bowl, toss cooked lentils with the apple, celery, parsley, lemon juice, remaining 2 tablespoons oil, and ¼ teaspoon each salt and pepper. Serve with the pork and any pan drippings.