Preheat oven to 400°F and lightly grease a 9-inch deep pie dish or oven-safe skillet.
In a large saucepan, melt vegan butter over medium heat, then add onion and cook for 5 minutes until softened, adding garlic for the last minute.
Sprinkle flour over the onion mixture and stir constantly for 2 minutes to create a roux.
Gradually whisk in vegetable broth and plant milk, stirring constantly until the mixture thickens into a gravy, about 5 minutes.
Stir in cooked lentils, mixed vegetables, thyme, sage, salt, and pepper, then cook for 5 minutes until vegetables are tender.
Pour the filling into the prepared dish, let cool for 5 minutes, then top with puff pastry, cutting slits for steam to escape.
Bake for 25-30 minutes until the pastry is golden brown and puffed, then let cool for 10 minutes before serving.