Preheat the oven to Gas Mark 7, 200°C, fan 180°C. Pulse the breadcrumbs and butter in a food processor for a minute. Add the cheese, nuts, garlic and herbs and pulse until combined. Spoon the mix into a greased 28cm (11in) flan dish and press into even layer. Bake for 10-15 minutes, until golden.
Melt the butter in a frying pan, then add the leeks and onions. Cook for 5 minutes until softened, then add the tomatoes. Add the flour and milk and stir. Simmer for 3 minutes and season.
Remove the pastry case from the oven and spoon in the leek mixture. Sprinkle over the breadcrumbs and bake at Gas Mark 4, 180°C, fan 160°C for 20 minutes. Serve with salad, if you like.
Freezing and defrosting guidelines
Once the dish has cooled completely, transfer it to an airtight, freezer-safe container, seal and freeze for up to 1-3 months. To serve, defrost thoroughly in the fridge overnight before reheating. Loosely cover with foil and bake until dish is thoroughly heated through. Reheat until piping hot.