1. Prep the potatoes

• Wash, peel, and thinly slice potatoes to about 2–3mm thickness using a sharp knife or mandoline slicer.
• Soak the slices in water briefly to remove excess starch, then drain and pat dry.

2. Make the cheese sauce

• In a medium saucepan, melt butter over medium heat.
• Add chopped onions and sauté until soft and translucent, about 3–4 minutes.
• Stir in flour to form a roux and cook for 1–2 minutes until lightly golden.
• Gradually whisk in milk and cream, stirring continuously to avoid lumps.
• Add salt, pepper, garlic powder, and thyme.
• Simmer until the sauce thickens slightly.
• Stir in cheddar and mozzarella cheeses until melted and smooth.

3. Layer the potatoes

• Preheat oven to 180°C (350°F). Grease a 23x33cm baking dish with butter.
• Spread a thin layer of cheese sauce on the bottom of the dish.
• Add a layer of potato slices, slightly overlapping.
• Pour more cheese sauce over the layer, spreading evenly.
• Repeat layering potatoes and sauce until all ingredients are used.
• Finish with a generous layer of cheese sauce on top.

4. Add the topping

• Sprinkle extra cheese over the top.
• Dot with small amounts of butter for a crisp, golden finish if desired.

5. Bake to perfection

• Cover with foil and bake for 40 minutes.
• Remove foil and bake for another 20–25 minutes until the top is bubbly and golden.
• Check for doneness by piercing potatoes with a knife — they should be tender all the way through.

6. Rest and serve

• Allow the dish to rest for 10 minutes before serving.
• Garnish with chopped parsley or chives and serve warm.