Cook macaroni in boiling, salted water for two minutes less than the packet instructions suggest. Drain & set aside.
Heat 1 tbsp butter in a saute pan and fry the bacon until it begins to brown. Add the finely sliced leeks and cook for a few minutes until they begin to soften. Add the frozen peas, give it a stir and set aside.
Meanwhile, heat the remaining butter in a saucepan until melted and bubbling. Stir in the flour and cook for one minute.
Remove from the heat and whisk in a little of the milk until combined. Slowly stir in the remaining milk and return to the heat, stirring until thickened.
Remove from the heat and add the grated cheeses, mustard and cayenne. Stir well until the cheese has melted.
To prepare the topping mix the ingredients together and sprinkle over the top of the macaroni and cheese.
Bake, in the centre of the oven, for 30 minutes.
Mum of three, daydream adventurer, OU Life Sciences graduate, occasional crafter, ex-pat Canadian & quite possibly Britain's most northerly food blogger.