Sift the flour and salt into a large mixing bowl. Make a well in the centre, break the eggs into it and whisk everything together.
Combine the milk and water in a jug then gradually add to the batter while whisking continuously. Once all the liquid has been added, continue to whisk until the batter is smooth and creamy. Whisk in the melted butter.
Grease an 18cm frying pan and heat until it is really hot, reduce the heat to medium and ladle about 2 tbsp of batter into the pan then immediately swirl around so the base is evenly covered. Cook until it begins to set then flip over and cook the other side briefly. Make 12 pancakes in this way, stacking them between sheets of greaseproof paper as they’re made.
Preheat the oven to 200°C/400°F and liberally butter a large baking dish.
Place the leeks in boiling, salted water, cook for about 3 minutes then cool in cold water. Squeeze dry on kitchen roll and cut each into four.
Add the spinach to the boiling water, removing it as soon as it collapses. Cool in cold water, squeeze dry and gently unfurl.
Place half a slice of ham in the middle of a pancake, add two pieces of leek and a little spinach and top with a piece of Brie or Camembert. Roll neatly and place on the baking dish. Repeat with the remaining pancakes.
Sprinkle with the grated cheese and bake for 25 minutes or until hot and bubbling.