Preheat the oven to 190°C, fan 170°C, gas 5. Heat the oil in a pan and cook the leeks for 5-10 minutes until softened and beginning to turn golden, adding in the garlic, fennel seeds and half the thyme for the last minute.
Stir through the cooked/ leftover turkey, crème fraîche, spinach and half of each of the cheeses. Bring to a gentle simmer, then remove from the heat.
Meanwhile, cook the pasta according to pack instructions; drain, reserving 150ml of the cooking water, then stir together with the turkey mixture and reserved water. Pour into a 1.5-litre dish.
In a bowl, mix together the remaining cheese, thyme leaves and breadcrumbs. Scatter over the top of the pasta and bake in the oven for 15 minutes until golden.