Cook the pasta until al dente, drain.
Melt the butter in a saucepan – and keep it going until it turns light brown, then add the flour and stir. Start adding the milk, bit by bit, whilst whisking. You may not need all the milk – you want a thick sauce, but not runny. Keep bringing to the boil to assess.
When the sauce is done, turn off the heat and stir in the cheddar and 30g of the brown cheese and combine until melted. Add the mustard, salt and pepper and taste – it will be quite sweet, so you need to add some vinegar, try 2 tbsp first and then if more is needed, add.
Add the pasta to the sauce and stir, then add to an oven proof bowl. Scatter the rest of the brown cheese on top and then add a thin layer of breadcrumb. Place in a hot oven for around 10-15 minutes until the top is crispy and bubbling and the brown cheese has melted.
Serve with a side salad. Pure comfort.