Preheat oven to 180°C (fan 160°C). Lightly grease a 10-12 cup capacity baking dish with non-stick cooking spray.
In a jug, whisk together the cream, garlic, chicken stock powder, salt and pepper until well combined. Set aside.
Layer one-third of the potatoes into the dish, slightly overlapping. Scatter over half the onion and bacon. Drizzle with a quarter of the cream mixture.
Repeat layer as above finishing off with remaining one-third of potatoes. Pour remaining cream evenly over the top. Cover with a layer of baking paper then a tight layer of foil.
Bake for 1 hour, allowing the potatoes to steam gently and soften.
Remove the foil and baking paper, scatter the cheeses evenly over the top, then return to the oven uncovered for 25–30 minutes until golden and bubbling and the potatoes are tender.
Rest for 10 minutes before serving so the bake can settle and slice more cleanly.