Heat the oven to 200°c /180°c fan
Make the pancake batter. Place the flour and salt in a large bowl. Make a well in the centre and crack in the eggs. Pour half the milk into the bowl and whisk until you have a thick, smooth batter. Pour in the remaining milk and whisk again. You’re aiming for the consistency of double cream. Cover the batter and refrigerate until needed.
Start the filling. Bring a pan of salted water to the boil. Add the leeks, bring back to the simmer and cook for 2-3 minutes, until just tender, then drain and set aside.
Heat the milk for the white sauce with the bay leaves, onion, celery, and peppercorns. When it begins to simmer, remove the pan from the heat and set aside to infuse.
When you’re ready to cook the pancakes, heat a large non-stick frying pan over a low heat. Add the knob of butter and, when it’s bubbling, pour nearly all of it into the bowl of batter and whisk. Add a small ladleful of the pancake batter to the pan, then tilt the pan to spread the mixture out evenly. As it sets, after about 1 minute, lift up the edge of the pancake with a spatula, then flip the pancake over. Cook the other side for 30-40 seconds, until golden, then slide the pancake out onto a warm plate. Repeat until you’ve made 6 pancakes in total.
Heat the butter for the white sauce in a medium, heavy-based pan over a medium heat. Scatter over the flour and, using a wooden spoon, work it into the butter. Cook gently, stirring regularly, for 1 minute. Set a sieve over the pan and pour in half the infused milk. Remove the sieve and work the milk into the butter and flour mixture. Once it thickens, replace the sieve, and pour in the remaining milk, then remove the sieve again and beat the sauce with a whisk to a thick, smooth consistency. Add half the cheddar cheese and all the crumbled blue cheese and stir well. Set aside a 3rd of the cheese sauce for later.
With the remaining 2/3 of the sauce, stir in the shards of ham, the leeks, and the chopped parsley. Season with plenty of salt and pepper
Lay out your pancakes on a large, clean work surface. Divide the leek and blue cheese mixture between them. Carefully roll up the pancakes and place them in a large baking dish side by side. Pour over the remaining white sauce and scatter over the remaining cheese
Bake in the oven for 25 minutes, or until golden brown on top and bubbling. Serve with a dressed green salad.