Bring potatoes and sweet potato to boil in a saucepan of water over medium-high heat. Cook for 15-20 minutes or until just tender. Drain. Set aside to cool. Peel potatoes. Thinly slice potato and sweet potato.
Meanwhile, heat half the oil in a frying pan over medium heat. Cook the bacon and onion for 5 minutes or until onion is soft. Transfer to a bowl. Heat remaining oil in pan. Cook garlic and mushroom for 3-5 minutes or until mushroom is tender. Add to the bowl. Stir in the chicken and spinach.
Melt butter in a saucepan over medium heat until foaming. Add flour. Stir for 1-2 minutes or until mixture bubbles. Remove from heat. Gradually whisk in milk. Stir over medium heat for 5 minutes or until sauce thickens. Stir in two-thirds of the parmesan. Season with salt and pepper. Add to bacon mixture, reserving 250ml (1 cup) sauce.
Brush a 3L (12-cup) capacity baking dish with melted butter. Place one-third of the potato, slightly overlapping, over the base. Top with half the sweet potato and half the chicken mixture. Continue layering with remaining potato, sweet potato and chicken mixture, finishing with potato. Top with reserved sauce. (To freeze, see note).
Preheat oven to 200ºC. Sprinkle with remaining parmesan. Bake for 50 minutes or until golden. Serve with broccolini.
Serving size: 6
Recipe courtesy of taste.com.au