1. Preheat your oven to 200°C/Fan 180°C/Gas 6. Remove the pastry from the fridge and pop to one side to warm up a little.
2. Trim the roots and woody greens off the leeks. Rinse out any grit, then thinly slice. Peel and thinly slice the garlic. Pour 1 tbsp oil into a large frying pan and warm over a medium heat. Slide the garlic and leeks into the pan. Season with a good pinch of salt and pepper. Fry the leeks for 8-10 mins, stirring occasionally, till soft.
3. Meanwhile, coarsely grate the cheddar and slide it into a mixing bowl. Add the crème fraîche and crack in the eggs. Spoon in the Dijon mustard and season with a pinch of salt and pepper. Whisk together with a fork, till well combined.
4. Line the bottom of a 20-22cm round tart tin or loose-bottomed cake tin with baking paper. No tart tin? Use a small roasting tin. Roll the pastry out thinly with a pinch of flour from your cupboard, or between two pieces of baking paper. Carefully line the base and sides of the tin with the pastry and gently press the it into the corners.
5. When the leeks are soft, take the pan off the heat and allow them to cool for 2-3 mins. Stir the cooled leeks into the egg mixture, then pour into the pastry case. Sprinkle the tart with a pinch of salt and pepper and slide it into the oven. Bake the tart for 30-35 mins, till the top is golden brown and the filling is just set.
6. When the tart is cooked, take it out of the oven and allow to cool for 10-15 mins, before carefully removing it from the tin. The filling will be softly set and oozing when warm, but if you prefer a firmer set filling, leave to cool for further 30 mins. Slice the tart, then divide betwen warm plates. Serve with a green salad on the side.
7. Love Your Leftovers
The cooked tart will keep in the fridge, whole or in slices, for up to 3 days. The tart is delicious served cold, or it can be reheated in the oven. Reheat the tart at 200°C/Fan 180°C/Gas 6 for 15-20 mins, till piping hot all the way through.