Whisk lemon juice, dill, mustard, salt and pepper in a large bowl. Gradually whisk in oil. Add lentils, feta, bell pepper, cucumber and onion; toss to coat.
Make Ahead Tip: The salad will keep, covered, in the refrigerator for up to 8 hours.
1 starch, 1/2 vegetable, 1 lean meat, 1/2 medium-fat meat, 2 1/2 fat