Preheat the oven to 220°C (200°C fan).

In a small bowl, mix together the breadcrumbs, thyme leaves and chopped hazelnuts. Set aside.

Bring a large pan of lightly salted water to the boil. Add the sprouts and cook for 3 minutes, then drain and set aside to steam-dry.

Melt the butter in a pan over a medium heat. Add the garlic and cook for 1 minute until fragrant, then add the leeks with a pinch of salt and cook for about 7-8 minutes until softened and beginning to caramelise, stirring occasionally.

Stir in the flour and cook for 2 minutes. Reduce the heat to low then whisk in the milk gradually to create a smooth sauce. Return to heat and simmer, whisking for another 2 minutes until thickened. Stir in the cream, mustard, butter beans with their liquid and grated cheddar until well combined. Stir in the sprouts until fully coated in the sauce. Season with salt and pepper.

Pour the mixture into an ovenproof dish and scatter this over the top of the gratin. Bake for 20-25 minutes until the topping is golden and the sauce is bubbling. Let it rest for a few minutes before serving.