STEP 1 Preheat the oven to 400 degrees F. In a large frying pan, cook the leeks, spinach and thyme in the vinaigrette, on medium heat for 5 to 6 minutes or until the leeks are tender, stirring frequently. STEP 2 Toggle in layers the sweet potatoes, the mixture of leeks, 1/2 cup of the grated cheese and the potatoes in a greased flat of 2 l ranging from the oven. Mix the milk, salt and pepper; pour over vegetables. Garnish with the rest of the grated cheese and the parmesan cheese. Cover. STEP 3 Bake in the oven 45 minutes; uncover and bake for another 15 minutes or until the vegetables are tender and the top is slightly browned.