1
Preheat oven to 375°F. Blanch the raw turnip slices in salted boiling water for 3-5 minutes. Remove from water and drain.
2
Coat the inside of the casserole dish with butter. [Omit if using a pie shell.]
3
Layer on half of the turnip slices in a single layer, season with salt and pepper. Layer on all of the onions. Sprinkle again with salt and pepper. Layer on the rest of the turnips. Set aside.
4
In a heavy sauce pot, heat the butter over a medium heat. Once the butter melts, slowly add in your flour and whisk. Its going to form a paste. Let the paste cook for 2-3 minutes, whisking constantly to prevent the roux from burning (this also cooks away that FLOUR taste and makes it nutty-tasting). With your whisk in hand, slowly pour in your milk. Lower your flame so as not to scald the milk. Whisk in a figure 8 motion. Season with salt and pepper.
5
Allow the bechamel sauce to simmer slightly over low heat for about 5-8 minutes and then add your cheese. Stir the cheese in using figure 8 motions.
6
Once the sauce is nice and thick (think alfredo sauce), pour sauce over the turnips.
7
Cover. And Bake for 20-25 minutes. Uncover and bake for 5 additional minutes
8
Remove from the oven and let cool for 10 minutes before serving.