Chop up the carrot, celery and leek into 1 cm cubes and fry them together in the oil in a large saucepan, along with the chopped garlic. Once they have softened (about 5 minutes) add the brown lentils and pour in the vegetable stock and water, stirring continuously. Bring to a boil then simmer.
Add the thyme, turmeric, paprika, garam masala and salt to the pan and leave to simmer for 20 minutes, or until much of the liquid has evaporated.
Remove the pan from the heat and stir in the breadcrumbs. Let the mixture cool for 10 minutes. Meanwhile, preheat the oven to 190°C/ 375°F/gas mark 5.
Lay the puff pastry out onto a greased/lined baking tray and spoon the mixture down the middle of the pastry in a cylindrical shape. Tuck the shorter sides in, as if you were wrapping a burrito, and then wrap the longer sides over to meet each other, overlapping slightly. Place the seam side down on the baking tray.
Using a knife, lightly score the top of the Wellington with diagonal crisscross lines, making sure not to go too deep. Brush the top of the pastry with the plant-based milk of your choice and sprinkle with poppy seeds.
Cook in the oven for about 30 minutes, or until the pastry starts to look golden.