Preheat the oven to 220C/425F/Gas 7.
For the pork, heat the oil in a non-stick frying pan. Season the pork fillet and sear in the hot pan for 1-2 minutes on each side. Transfer to the oven and cook for 8-10 minutes, or until cooked through.
For the leeks, heat the oil and butter in a pan and sauté the garlic with the leek and thyme leaves for 3-4 minutes, to soften.
Stir in the milk, cream and parsley, then reduce the heat and simmer gently for a further 6-8 minutes, stirring occasionally.
Remove the pork from the oven and allow to rest. Slice on the diagonal. Spoon the creamed leeks onto a serving plate and top with the sliced pork.
Serve.