Preheat the oven to gas 4, 180°C, fan 160°C. Cook the Brussels sprouts in a large pan of boiling salted water for 3 minutes, drain and refresh in iced water. Lightly butter a large 28cm x 22cm oven-proof dish and add the sprouts.
Fry the bacon pieces in a frying pan over a medium heat until crisp. set aside. Mix together the crème fraîche , mustard and milk and pour over the sprouts. scatter with the gruyere and the bacon. Bake for 15- 20 minutes, until bubbling and golden. Serve immediately.
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