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1
Preheat the oven to 400 F or 200 degrees C.
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2
Peel the potatoes and slice thinly.
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3
Wash the starch off them and then pat dry in a cloth.
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4
Trim the leeks, discarding the coarse green ends, then slice thinly.
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5
Butter a gratin dish and put in half the potatoes and top with all the leeks.
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6
Cut the bacon into strips, fry until crisp, and then put the bacon over the leeks.
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7
Briefly fry the sage leaves in the leftover bacon fat until browned and crisp.
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8
Scatter over the leeks, along with the bacon fat.
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9
Grate the cheese directly over the top of the dish and then cover with the rest of the potatoes.
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10
Heat the stock and pour it over the potatoes until it reaches the top layer, but do not submerge them.
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11
Season with salt and pepper and dot with most of the remaining butter.
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12
Smear the last of the butter on a sheet of waxed paper and lay it, buttered side down, on top of the dish.
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13
Cook for 45 minutes.
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14
Remove and check with a skewer that the potatoes are nearly cooked.
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15
Mix the mustard with the creme fraiche and pour it over the gratin, smoothing with a rubber spatula to spread it evenly.
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16
Place the dish back in the oven without the paper for a further 15 minutes or until it is golden, bubbling and cooked through.
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17
Serve immediately.
