Preheat the oven to 200 degrees Celsius / 390 Fahrenheit.
To make caramelised onion, add oil to a pot and heat on medium. Add sliced onions and cook gently for 2 minutes, until softened.
Add brown sugar and ½ tsp salt. Stir to combine. Continue cooking gently on low to medium heat, stirring occasionally, for a further ten minutes. Set aside once finished. If you are using store bought caramelised onion, skip this step.
Meanwhile, while onions are cooking, slice the potatoes as thinly as possible into rounds.
In another pot, combine all remaining ingredients. Bring to the boil, stirring occasionally. Turn off the heat and set aside.
Brush a skillet or shallow baking dish with some olive oil. Place the potato slices evenly across the bottom of the skillet in layers. After 3 layers, pour in enough of the milk and quinoa mixture to cover the potatoes. Spoon over the caramelised onion. Layer the rest of the potatoes and pour in the remaining quinoa mix.
Tap the skillet gently on the bench to remove any bubbles and spread the liquid through the dish evenly.
Place in the preheated oven for approximately 1 hour, or until potatoes feel tender when tested with a sharp knife. Check after 40 minutes and cover with a lid or foil if it’s getting very browned.