First cook the potatoes in a pan of boiling water until softened, then drain.
Steam the broccoli until tender then drain.
Preheat the oven to 190 degrees C and lightly grease an ovenproof dish.
Heat the oil in a pan and cook the leeks and garlic for 10 minutes until softened, then set aside.
Place the milk in the same pan and bring to just under the boil. Mix the cornflour with 2 tablespoons water to form a paste, then whisk in and add the herbs and seasoning. Cook for a few minutes until thickened, then remove from the heat.
Stir in most of the Sheese and spinach leaves and stir to allow the spinach to wilt and Sheese to melt.
Arrange the cooked potatoes and broccoli in the dish in a single layer, then pour over the sauce to cover. Sprinkle the rest of the Sheese over then bake in the oven for 10-15 minutes, until bubbling.