Heat the olive oil over a medium heat before adding the leeks and garlic, cook them until lightly caramlised then add the thyme. Add the cream and simmer before seasoning with salt and freshly ground black pepper.
Add some of the vegetable to the base of a large roasting dish, season lightly and add some of the leek and cream. Continue layering the vegetables and cream mixture while seasoning. Add the layered dish to an oven preheated at 180 to cook for 30 – 40 minutes or until the vegetables are tender.
Sprinkle the crumble over and place it back in the oven to cook for another 10 minutes or until the top is crisp and golden.