Start by prepping your vegetables. Peel and roughly chop the onion. Remove the outer leaves from the cauliflower and cut into small florets, about 1-2 inches in size.
Put a large pan on a medium heat. Add the butter and melt. Add the flour and whisk until smooth. Remove from the heat.
Gradually add the milk, whisking all the time. Put back on a medium heat and continue whisking until the sauce thickens. Remove from the heat again.
Add the onion, bay leaf, mustard and 150g of the cheese to the sauce with a pinch of black pepper. Stir until the cheese has melted.
Lay the cauliflower florets over the base of an ovenproof dish. Pour over the sauce, then stir gently to make sure all of the cauliflower is coated in the sauce. Top with the remaining cheese and put in the oven for 40 minutes, until the cauliflower is cooked through and the cheese is golden brown.