In a medium saucepan of boiling salted water, cover and cook sweet potatoes over medium heat until very tender, about 15 min. Drain well. Mash well; stir in milk, butter, salt and pepper.
Preheat oven to 350°F (190°C). Stir together lentils, mixed vegetables, tomato sauce, cornstarch, thyme and garlic; spoon into two 1 1/2 cup (375 mL) baking dishes. Top with sweet potato mixture. Bake 35 to 40 min., or until golden and bubbling.
Triple this recipe to make a freezable 6-serving dish: Spoon lentil mixture into 13 x 9-in. (3 L) baking dish; top with sweet potato mixture. Let cool to room temperature. Wrap tightly and freeze for up to 2 months. Thaw in the fridge overnight before baking in 350°F (190°C) oven, 40 to 45 min, or until golden and bubbling.