Preheat oven to 375 degrees. Grease a medium sized baking dish and set aside.

Prepare the turnips by slicing off the ends and peeling the outer layer. Rinse off any dirt.

Using a mandoline, carefully slice each turnip into thin slices, about 1/4" in thickness. The total weight of the sliced turnips should be roughly 1 pound.

Layer the turnip slices in the prepared dish. They will overlap. Try to get the layers as even as possible so the top does not dome up. Set aside

In a small sauce pan over medium heat, heat the heavy cream but not let it boil. Whisk continuously to avoid burning.

Whisk in the dijon, horseradish, and garlic. Add a pinch of salt and pepper and whisk to combine.

When the cream is hot, after 3 to 4 minutes, add half of the white cheddar, half of the gruyere, and all of the Boursin. Whisk until all the cheese is melted. Remove from heat.

Carefully pour the cheese sauce over the turnips so that it is evenly coated. Use a wooden spoon to push the turnip slices around and make sure the sauce is evenly distributed throughout the dish.

Top the dish with the remaining white cheddar and gruyere cheese. Cover with foil.

Bake for 45 minutes covered. Remove the foil and cook an additional 10 minutes to get a nice golden color on top. Let the dish rest 10 minutes before serving.