Bake can be cooked up to two hours ahead. Stand, covered at room temperature. To reheat, cover with foil and place in a moderately slow oven (160C) for about 30 to 45 minutes, or until hot in the centre.
1 clove garlic, peeled, cut in half
200g block Swiss cheese
½ cup shredded parmesan
2 tablespoons finely chopped fresh thyme
1.5kg brushed potatoes, peeled
600ml carton pure cream
2 teaspoons Dijon mustard
¼ teaspoon ground nutmeg
Cayenne pepper, to garnish (optional)
Lightly grease a large ovenproof dish (14-cup capacity, 6cm deep).
Rub cut sides of garlic over the base and around side of dish. Place dish on an oven tray.
Coarsely grate Swiss cheese, you will need I cup (100g) Reserve remaining for another use. Combine Swiss cheese with parmesan and thyme in a bowl.
Using a mandolin and keeping them separate, very thinly slice potatoes and onions (about 2mm).
Spread one-third of the potatoes, in an even layer, over base of prepared dish. Top evenly with one-third of the onions. Sprinkle with one-third of the cheese mixture. Repeat layering with remaining potatoes, onions and cheese mixture.
Whisk cream, mustard and nutmeg in a large jug until combined. Season with salt and pepper. Pour evenly over cheese in dish. Cover with greased foil.
Cook in a moderate oven (180C) for 1 hour and 30 minutes. Remove foil. Cook, uncovered, in same moderate oven for a further 20 to 30 minutes, or until top is golden brown and potato is tender when a small sharp knife is inserted through the centre. Remove from oven. Serve.