Creamy Celeriac Gratin

Ingredients

• 1kg potato, peeled and thinly sliced

• 1 celeriac, peeled and thinly sliced

• 1 onion, peeled and sliced

• 1 garlic clove, peeled and crushed

• 1 knob of butter

• 284ml Jersey double cream

• 200ml Jersey whole milk

• 50g parmesan or your cheese of choice, grated

• 1 teaspoon wholegrain mustard