Peel, wash and halve the celery and cut into slices about 5 mm thick. Cook in plenty of boiling, lightly salted water for about 5 minutes.
Then drain and leave to drain. Also cut the beet into slices about 5 mm thick. Clean the leek, cut lengthwise, rinse thoroughly and cut into thin rings.
Peel the garlic clove and chop finely. Finely grate the mountain cheese. Butter a baking dish. Layer the celery and beet in it like a roof tile and season well with a little salt and a few twists of black pepper from the mill. Preheat the oven to 200° (convection oven 180°). Melt 25 g butter in a saucepan and sauté the leek rings – except for 1 tablespoon – with the garlic for about 3 to 4 minutes. Then take them out.
Add the remaining butter to the pot, melt over low heat, then stir in the flour and sweat for about 1 minute. Then pour in the milk, stir and bring to a boil. Stir the leek rings, horseradish and 2/3 of the grated mountain cheese into the béchamel sauce and season the sauce with a little salt, a few turns of black pepper and nutmeg. Spread the béchamel sauce on the vegetables and sprinkle the remaining grated mountain cheese on top. Bake the gratin in the preheated oven for about 20 to 25 minutes. Serve sprinkled with the remaining leek rings. Serve with mashed potatoes or fresh baguette.
