Preheat the oven to 200 degrees C and grease a baking dish of approx 23cm x 30c in size
Fry the leeks and garlic with a good glug of olive oil over a medium heat for about 5 minutes
Layer half the parsnips, potatoes and all the leeks in the baking dish and sprinkle with half of the cheese, reserving the bigger slices of parsnips and potatoes for the final layer
Press down firmly and place a last layer of parsnip and potato slices in overlapping rows and press down again
Using the same pot you used for the leeks, heat the cream and whisk in the flour, salt, nutmeg and black pepper
When the cream just comes to a simmer, pour over the layered vegetable, slanting the baking dish to all side to get the cream in all over the slices
Sprinkle the remaining cheese over the top, dot with a few leaves of thyme
Cover securely with foil and bake for 60 minutes
Remove the foil and pierce with a fork. The parsnips should be al dente (If not place back in the oven (covered) for another 15 minutes until al dente)
Remove the foil (don’t worry if the sauce looks watery and thin) and bake uncovered for 15 minutes until golden and bubbly
Let the gratin sit for 15 minutes to set and thicken before serving