Deluxe Potato Gratin

Directions

1. 1
   Preheat the oven to 400 F or 200 degrees C.
2. 2
   Peel the potatoes and slice thinly.
3. 3
   Wash the starch off them and then pat dry in a cloth.
4. 4
   Trim the leeks, discarding the coarse green ends, then slice thinly.
5. 5
   Butter a gratin dish and put in half the potatoes and top with all the leeks.
6. 6
   Cut the bacon into strips, fry until crisp, and then put the bacon over the leeks.
7. 7
   Briefly fry the sage leaves in the leftover bacon fat until browned and crisp.
8. 8
   Scatter over the leeks, along with the bacon fat.
9. 9
   Grate the cheese directly over the top of the dish and then cover with the rest of the potatoes.
10. 10
    Heat the stock and pour it over the potatoes until it reaches the top layer, but do not submerge them.
11. 11
    Season with salt and pepper and dot with most of the remaining butter.
12. 12
    Smear the last of the butter on a sheet of waxed paper and lay it, buttered side down, on top of the dish.
13. 13
    Cook for 45 minutes.
14. 14
    Remove and check with a skewer that the potatoes are nearly cooked.
15. 15
    Mix the mustard with the creme fraiche and pour it over the gratin, smoothing with a rubber spatula to spread it evenly.
16. 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.
17. 17
    Serve immediately.