Prepare the batter: place the flour, egg, milk and a pinch of salt into a food processor and blend until smooth, or put them in a bowl and blend with a hand-held mixer or a whisk. Allow the batter to stand while you fry the meat. Heat the oven to 190°C (375°F, gas mark 5).
To make the filling, dry-fry the minced pork in a nonstick saucepan or lidded frying pan over a medium heat until browned and crumbly. Add the leeks, mushrooms, cayenne pepper and sage. Cover, reduce the heat and let the mixture simmer, stirring occasionally, for 15 minutes, adding a little water if it becomes too dry.
Heat a small nonstick frying pan over a medium heat and brush with a little vegetable oil. Swirl 2 tablespoons of the batter into the hot pan and quickly tilt it to coat the base with a thin layer. Cook the pancake until lightly set, flip it over with a palette knife and cook it for a further 30 seconds. Remove it from the pan and keep it warm under a clean tea towel. Make seven more pancakes the same way, greasing the pan as necessary.
To complete the filling, blend the cornflour and mustard with 2 tablespoons of the milk. Bring the remaining milk to the boil in a large saucepan, briskly stir in the cornflour mixture and continue to boil until it is thickened. Season to taste.
Drain off any cooking juices from the pork, stir in the sauce and adjust the seasoning if necessary.
Spoon a strip of filling down one side of each pancake. Then roll them up and fit them snugly, seam side down, in a lightly oiled, shallow ovenproof dish. Arrange the tomato slices over the top and sprinkle with the cheese.
Bake the pancakes for 15-20 minutes until the tomatoes have softened and the pancake edges are just crisp. Serve garnished with a little chopped parsley, and Parmesan, if wished.