1. Peel the turnips.
2. Slice them and immerse them in boiling salted water for 4 minutes.
3. Drain and refresh them.
4. Melt 40 g of butter in a casserole dish, add the turnips, salt, and stew (covered) for 20 minutes over low heat.
5. Generously butter a gratin dish.
6. Preheat the oven to 240°C (thermostat 8).
7. Meanwhile, prepare the Mornay sauce (prepared like a béchamel sauce, but with added egg yolks).
8. Preparation of the Mornay sauce:
9. In a saucepan over low heat, melt the butter.
10. Add the flour.
11. Using a wooden spatula, mix until well combined.
12. Continue to mix until it thickens slightly.
13. Add the cold milk.
14. Season with salt and pepper.
15. Continue to mix over low heat until the sauce thickens.
16. In a bowl, mix the egg yolks with the cream.
17. Off the heat, add this mixture to the sauce while whisking vigorously.
18. Return to low heat and add 70 g of Leerdammer cheese, sliced.
19. Mix until the cheese is melted and incorporated into the sauce.
20. Arrange the turnips in the gratin dish. Smooth the top.
21. Cover with Mornay sauce.
22. Sprinkle with 30 g of Leerdammer cheese, thinly sliced.
23. Place the dish under the broiler until the top is golden brown.