Place potatoes in a medium saucepan. Cover with water; add ½ teaspoon salt. Bring to a boil; reduce heat. Cover and gently boil for 10 to 12 minutes or until fork-tender. Drain well.

Return potatoes to saucepan. Cook and stir over low heat for 30 to 60 seconds or until dry. Add butter; mash with a potato masher until smooth. Stir in enough milk until light and fluffy; season to taste with salt and pepper. Set mashed potatoes aside.

Meanwhile, sort lentils and place in a wire mesh sieve. Rinse under cold running water; drain well. Transfer to a small saucepan. Add 2 cups vegetable stock. Bring to a boil; reduce heat to medium-low. Simmer with lid tilted for 15 to 20 minutes or just until tender but not mushy. Remove from heat; uncover to cool. Do not drain.

Preheat oven to 400°. Lightly spray a 2-quart baking dish with nonstick spray; set aside.

Heat olive oil in a large nonstick skillet over medium heat. Add onion; cook for 2 to 3 minutes or until softened, stirring occasionally. Stir in garlic; cook for 30 seconds.

Stir tomato paste, thyme, ½ teaspoon black pepper and vinegar into onions. Add frozen mixed vegetables. Whisk together remaining 1 cup vegetable stock and corn starch; stir into mixture in skillet. Simmer for 4 to 6 minutes or just until vegetables are tender, stirring occasionally. Stir in lentils.

Transfer lentil mixture to prepared baking dish. Spoon mashed potatoes on top in small spoonfuls; spread to cover. Use back of spoon to make small peaks. Sprinkle with cheese.

Bake for 15 to 20 minutes or until heated through and slightly bubbly around edges. Remove from oven. Let stand for 10 minutes before serving. Garnish with chives, if desired.