Donna Hay's Sweet Potato, Leek and Sage Gratin

Here's my second Donna Hay recipe and it's a perfect potato dish for the holidays, although my photograph below doesn't look very Christmasy. I gave this a test run ahead of the holidays and loved the dish so much, I intend to make it for Christmas Eve dinner.
Use your mandolin to slice the potatoes...if you don't have one, slice thinly. I used fontina as it's such a nice melting cheese. Wonder how gruyere would taste?

Sweet Potato, Leek and Sage Gratin
Donna Hay, Issue 60

Ingredients:

40g (about 3 tablespoons) butter

4 leeks, cleaned and sliced

2 cloves garlic, crushed

sea salt and cracked black pepper

800g ( about 1 3/4 pounds; I used 2 large.) sweet potato, peeled and thinly sliced

800g (about 1 3/4 pounds; I used 2 large Idahos, they worked fine.) Desiree (waxy) potatoes, peeled and thinly sliced

2 tablespoons olive oil

1 cup sage leaves, torn in pieces

300g fontina or cheddar cheese, sliced

Method:

Preheat oven to 400 degrees F. 

Melt the butter in a non-stick frying pan over medium heat. Add the leek and garlic, salt and pepper and cook for 5 minutes or until softened. Place the sweet potato, potato, oil and salt and pepper in a bowl and toss to combine. Layer the sweet potato, potato, leek, sage and cheese in a 10 by 13-inch baking pan lined with non-stick baking paper, finishing with a layer of cheese. Cover loosely with aluminum foil and bake for 30 minutes. Remove foil and bake for a further 30 minutes or until golden and cooked through. Serves 6-8.