Potato and Leek Gratin with Prosciutto

There's nothing more comforting and festive than this cheesy, creamy gratin.

unsalted butter (50 g)
2 Tbsp. extra-virgin olive oil, plus extra to brush
2 leeks, finely chopped
2 cloves garlic, finely chopped
all-purpose flour, unbleached (40 g)
milk (450 mL)
Parmesan cheese, finely grated (40 g)
⅓ cup sage leaves, chopped
Dutch cream potato, peeled and thinly sliced on a mandoline (400 g)
8 thin slices prosciutto
¼ cup Gruyère, finely grated

Melt butter and oil in a non-stick saucepan over medium-low heat. Add leek and a pinch of salt flakes. Cook, stirring occasionally, for 15 minutes or until leek is very soft. Stir in garlic and cook for 2 minutes or until softened. Stir in flour and cook for 2 minutes to cook out the flour. Gradually add milk, stirring constantly, until thick and smooth. Stir in Parmesan and chopped sage, and season to taste.

Preheat an oven to 350°F (180°C/160°C fan, Gas Mark 4). Place two 18 cm ovenproof frypans on a baking tray.

Divide one-quarter of the leek béchamel between the bases of the pans. Layer each pan with one-quarter of the potato and top with 2 slices of prosciutto. Repeat layering, finishing with a layer of béchamel and prosciutto. Brush with oil.

Cover each pan with a layer of baking paper, then foil. Bake for 45-50 minutes, until the potato is tender.

Remove the baking paper and foil and bake for a further 10 minutes. Sprinkle over the Gruyère, top with extra sage leaves and bake for a further 5-10 minutes, or until golden and bubbling. Serve.