Melt 30 grams of the butter in a large pan over a medium heat. Add the gnocchi and cook, stirring regularly, for about 5 minutes until golden brown. Tip into a large ovenproof dish.
Melt the rest of the butter in the pan. Add the flour and stir to create a thick paste, then turn off the heat. Gradually add the milk, stirring all the time, to make a smooth sauce. Add the bay leaves, mustard and salt and pepper. Turn the heat back to medium and cook until the sauce is simmering and thick.
Cut the broccoli into small florets and add to the sauce, along with 150g of the cheese. Stir until combined.
Tip the sauce into the dish with the gnocchi, stir until the gnocchi is all coated in the sauce. Top with the remaining cheese.
Bake in the oven for 30 minutes, until golden and bubbling.