Cut the brussels sprouts into half and remove the tough ends.
Rinse the leeks well with plenty of cold water to remove any dirt that might hide and chop them.
Peel and chop the onion and garlic cloves.
In a large pan, heat up the oil and add the butter - allow it to melt.
Add the chopped onion and fry until golden, then add the garlic, leeks and brussels sprouts and sautee for 5 minutes.
Pour over the water, place a lid on the pan, and leave to cook until the water is evaporated and the veggies are tender.
In a separate pan, make the bechamel sauce.
Melt the butter, add the flour and whisk well to get a smooth paste.
Pour over the milk, and continue to whisk until the sauce thickens.
Add the cheeses, dijon mustard and seasoning and mix well to get a cheesy sauce.
Pour the sauce over the veggies and mix well to combine.
Transfer everything to an ovenproof dish and top with the breadcrumbs and remaining grated cheese.
Bake in the preheated oven at 200 degrees Celsius (390 Fahrenheit) for 20-25 minutes until golden and bubbling hot.