Roll out one circle of the pastry dough and place it into the bottom of a 9-inch pie plate.

Pre-heat a 12-inch skillet over medium-high heat. Add 2 tablespoons of the butter and the olive oil and sweat the onions and carrots for 5 minutes. Add the butternut squash, celery, herbs de Provence, sage, salt and freshly ground black pepper. Continue to cook for 5 to 10 minutes, until the vegetables start to soften. Add the leeks and sauté for another few minutes.

Sprinkle the flour over the vegetables, stir and cook for one minute. Add the sherry and vegetable stock and simmer until the mixture thickens. Add the lentils and simmer over low heat for another minute or two. Remove the pan from the heat and stir in the kale. Let the mixture cool for at least 5 minutes before transferring it to the pastry-lined pie plate. Dot with the remaining tablespoon of butter.

Roll out the second pastry circle and place it on top of the pie. Pinch the edges shut to form a decorative crust. Brush the top of the pie dough with egg wash and then make 4 slits in the top. Refrigerate the pie for at least 20 minutes.

Pre-heat the oven to 350°F.

Transfer the pie to the oven and bake for 60 to 70 minutes, until the pastry is golden brown, and the filling is bubbling. If the crust gets too brown, cover the edges with some foil.