Preheat the slow cooker if necessary; see the manufacturer's instructions. Butter the inside of the slow cooker pot, then add the shallots or onion and garlic.
Put the cornflour in a small bowl and mix with a little of the beer to make a smooth paste, then blend with the remaining beer. Add to the slow cooker with both cheeses, a little nutmeg and some salt and pepper.
Stir together, then cover with the lid and cook on high for 40–60 minutes, whisking once during cooking. Whisk again and serve with the dippers arranged on a serving plate, with long fondue or ordinary forks for dunking the dippers into the fondue.