Whisk 3 tablespoons oil, vinegar, mustard, salt and pepper in a large bowl.
Heat the remaining 1 tablespoon oil in a large skillet over medium-high heat. Add garlic.
Cook 30 seconds. Stir in lentils and heat through, about 2 minutes. Stir in 5 tablespoons of the dressing and remove from the heat. Stir in fennel, apple and celery.
Toss greens with the remaining dressing. Serve with the warm lentil mixture on top.
Notes
Tip: To cook lentils, place in a saucepan, cover with at least 1 inch of water, bring to a simmer and cook until just tender, 15 to 30 minutes, depending on the type of lentil.Drain and rinse with cold water. 1 cup dry lentils = about 2 1/2 cups cooked. Or use canned lentils: 15-ounce can = 1 1/2 cups. Rinse canned lentils before cooking with them to reduce the sodium by about 35%.