Cook the potatoes in lightly salted boiling water for 10 minutes, then drain.
Melt the butter with the oil in a medium saucepan and cook the onion for about 5 minutes, or until soft and golden. Add the garlic and cook for a further 2 minutes.
Add the grated cheese and cream or crème fraîche. Stir until the mixture is hot and the cheese has melted. Season to taste.
Arrange half the potatoes in a lightly buttered, shallow, ovenproof dish. Place half the cheese slices over the potatoes and pour over half the cheese sauce. Cover with the remaining potato slices, the rest of the cheese sauce and top with the remaining cheese slices.
Cook in a preheated oven, 220°C (425°F), Gas Mark 7, for 30–35 minutes until bubbling and golden brown. Serve immediately with the chicory and dressing.