Preparation
1. Prepare the potatoes
Peel, wash and slice the potatoes. Cook them for 10 to 12 minutes in a large saucepan of boiling salted water until soft. Carefully drain the water.
2. Prepare the filling
Cut the ham. If using, sauté the onion and garlic in a little butter or oil until softened. Add thyme if desired.
3. Make the sauce
Melt the butter, add the flour and stir to form a roux. Gradually whisk in the milk. Let thicken, then add the crème fraîche and season with salt, pepper and nutmeg.
4. Assemble the gratin
Preheat the oven to 180°C (350°F). Grease a gratin dish. Alternate layers of potatoes, ham, onion/garlic (if using) and sauce. Finish with grated cheese.
5. Bake
Bake for 30 to 40 minutes, until the top is golden brown. Leave to stand for 5 minutes before serving.
