If you can’t find y fenni cheese, you can add a teaspoon of mustard to the cheese sauce for a piquant mustard flavour.
Preheat the oven to 180C/350F/Gas 4.
For the filling, heat the butter and oil in a lidded saucepan over a low heat until melted. Add the chopped leeks and cook for 4-5 minutes, or until soft, taking care that they don’t brown. Once soft, add the asparagus stems to the pan, pour the water over and cover with a lid. Simmer for 3-4 minutes over a medium heat, and then cool. Set aside.
For the pancake batter, sift the flour into a bowl and add the salt. Whisk the milk and eggs together in a jug then whisk into the flour mixture. Add the olive oil and cooled melted butter and whisk well. Pour the batter into a jug and leave to stand in the fridge.
For the cheese sauce, warm the milk and set to one side. Melt the butter in a pan. Once melted, remove from the heat, stir in the flour, return to the heat and cook for 1-2 minutes. Remove the pan from the heat and slowly add the warm milk, stirring continuously. Return the pan to the heat and cook the sauce for 5-10 minutes, stirring constantly, until thick and smooth. Stir in the grated cheese and set aside.
Heat a non-stick frying pan over a medium-high heat. Brush the pan with oil. Pour a ladleful of batter into the pan and swirl around until the base of the pan is covered. Cook for 1–2 minutes until the pancake has released from the base of the pan and is beginning to brown, then flip over and cook for about a minute on the second side. Slide the pancake from the pan onto a plate, cover and set aside. Continue to cook the pancakes until all the mixture is used up, putting sheets of greaseproof paper between each pancake on the plate so that they don’t stick together.
To assemble the pancakes, take one of the pancakes and add a spoonful of the filling to the middle. Pour over 1-2 tablespoons of cheese sauce, then roll up neatly and place in an ovenproof dish. Continue until all the pancakes and filling are used up, then pour the remaining cheese sauce over the dish. Bake for 20 minutes until piping hot and golden on top.
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