Place lentils in a medium saucepan and add enough water to cover by about 2 inches. Bring to a simmer over medium heat. Reduce heat to maintain a gentle simmer; cover and cook until tender, 20 to 25 minutes. Drain and rinse with cold water until cool.
Meanwhile, combine sliced shallot, minced garlic, 2 teaspoons lemon zest and ¼ cup lemon juice in a large bowl; let stand for 15 minutes, stirring occasionally. Whisk in 6 tablespoons oil, 2 tablespoons mustard, 1 teaspoon salt, 1 teaspoon pepper and 1 teaspoon honey until combined.
Add 2 cups tomatoes, 2 cups cucumber, 1 cup carrots, 1 cup celery, the lentils and ½ cup herbs; toss to combine.
