Preheat oven to 400°F; lightly grease a 9-inch deep pie dish or oven-safe skillet.
Melt butter in large saucepan over medium; cook onion 5 minutes until softened, adding garlic in last minute.
Sprinkle with flour; stir constantly 2 minutes to form roux.
Gradually whisk in broth and milk; stir constantly about 5 minutes until thickened into gravy.
Stir in lentils, vegetables, thyme, sage, salt, and pepper; cook 5 minutes until vegetables are tender.
Pour filling into dish; cool 5 minutes, then top with pastry and cut steam vents.
Bake 25-30 minutes until pastry is golden brown and puffed; cool 10 minutes before serving.