This is a relatively healthy gratin but if you want it richer you can pour half a cup of cream over the top before baking.

Clean 1 leek and slice in thin half rounds.  Spread the leeks on the bottom of a 9″ square baking dish or cast iron skillet brushed with 1 T. olive oil.  Sprinkle with 1 T. chopped fresh rosemary, salt and pepper.

Very thinly slice 1 lb. of potatoes.  Make a layer of potatoes on top of the leeks.  Sprinkle with salt and pepper.

Slice 1/2 lb. turnips thinly, but not as thinly as the potatoes.  Make a layer atop the potatoes.  Sprinkle with salt and pepper and drizzle with 1 t. olive oil.

Grate 8 oz. gruyere cheese.  Make a thin layer atop the turnips.

Layer the remaining vegetables, sprinkling with salt and pepper and drizzling with a little more olive oil.

Top with the remainder of the cheese.

Bake the gratin at 350 degrees until the top and edges are nicely browned.