Clean and finely slice the leek. Peel the carrot and cut it into fine (julienne) strips.
Heat 25 grams (1 tablespoon) butter and 1 tablespoon of oil in a large pan, add the leek and carrot and cook on a low heat for about 8 minutes with the lid on, stir occasionally. You want the leek and carrots to soften but not colour.
Turn the heat up to medium and add 1 tbsp flour, cook the flour for about five minutes, stirring.
Slowly add the 150 ml (¼ pint) of vegetable stock, stirring all the time, to make a thick sauce, bring to a bubbling simmer and cook for about 3-4 minutes.
Add 1 tbsp cream cheese, 250 g (9 oz) spinach and ¼ tsp garlic granules. Replace the lid and let the spinach steam, once it has reduced stir it into the sauce. The moisture from the spinach will help to make the sauce slightly thinner.
Take the pan off the heat and add the 150 g (1⅓ cups) grated cheese, stir through the sauce and set aside to cool.
Pre-heat the oven to 220℃/200℃ fan/400℉
Butter a 27 cm (10 in) pie dish and fill with the sauce, spreading it evenly..
Cut the 320 g (11 oz) pastry sheet into 1.5 cm (½ in) strips across the short edge.
Brush the edge of the pie dish with beaten egg and stretch two strips to fit round the edge of the pie dish, then brush the top of the pastry strip with beaten egg.
Attach the pastry strips to one edge of the pie, then add more strips at a 90 degree angle, then cross them over and under each other to make the lattice. If the pastry breaks or you have to join two pieces together you can cover the joins with circles of pastry cut with the end of an icing (frosting) nozzle or other small cutter.
Brush the lattice with beaten egg and sprinkle with 1 tbsp sesame seeds, if using.
Bake for 30-40 minutes until golden brown and serve with potatoes and steamed green vegetables or a fresh green salad.