Preheat oven to 180 degrees Celsius (350 degrees Fahrenheit)
Slice the sweet potatoes either using a mandoline, the slicer setting on a food processor, or very thinly slice them by hand (no need to peel them)
Place the sliced sweet potatoes potatoes in a large baking dish
Heat up oil in saucepan and add onions once hot
Fry on a low heat for around 10 minutes, until soft and slightly caramelised
Measure out the almond milk, and add the cornflour to the jug or measuring container and mix very well until completely dissolved
Add this almond milk + cornflour mixture, nutritional yeast, stock cube, salt + pepper and bring to a boil then turn to a low heat
Keep stirring until the sauce has thickened - it may seem very thick at this stage, but the sweet potatoes do release water whilst cooking so the sauce should definitely be on the thicker side
Pour the sauce over the sliced sweet potatoes
Cover the dish with tin foil or baking paper and bake for around 30 minutes
Take off foil or baking paper and bake for a further 15 minutes, until the top is crispy and golden brown