Preheat the oven to 180 C/ Gas 4. Add 25g of the butter to a large frying pan and set to a medium heat. Chop the leeks and add to the pan. Fry until softened. Place the leeks in an oven proof dish and place to one side.
Melt the remaining butter in a medium sized saucepan. Add the flour and cook for 5 minutes, keep stirring. Whisk in the stock and milk and cook until the sauce thickens.
Add both mustards along with the smoked cheese. Stir through until combined. Pour the sauce over the cooked leeks.
In a bowl toss the bread with the olive oil – make sure it is completely coated. Add the smoked paprika, garlic powder and dried thyme and mix together. Sprinkle the bread mixture on top of the leeks in sauce.
Place in the oven and bake until the topping is golden and the sauce is bubbling, about 20 minutes.