Place a large shallow pan onto a low to medium heat and add the butter, once melted add the sliced leeks, ½ the salt and pepper, cover with a lid and cook for 2-3 minutes with no colour then add the shallots, garlic, a tsp of thyme leaves, cook for 2-3 minutes with no colour.
Then add the white wine and reduce till the pan is dry, add the white sauce and bring to a simmer then remove from the heat and add the spinach leaf. Stir in till it has cooked down then add the fresh chopped herbs, peas and cheese and remaining seasoning.
Take the courgettes and slice them very thinly on a mandolin approx 1mm thick. Take a large flat tray with 2 inch deep sides and brush it with a little olive oil. Then lay the PASTA along the bottom of the tray , then ½ the courgette and season with a little course sea salt and milled pepper, lay on the spinach filling and continue with another 2 layers, seasoning each layer.
Finish with a layer of courgette and then cover in the grated cheese and dried rosemary. Place into the oven at 180c for 20 minutes and serve instantly.