In a pot, melt vegan butter, then add flour and mix to combine. Whisk lukewarm milk slowly, keep whisking until it reaches a thick consistency. Then season with salt, pepper, nutmeg and nutritional yeast and mix to combine.
Pour a ladle of béchamel on the bottom of a baking dish. Add a first layer of potatoes, placing one slice overlapping the other. Add some onions, salt, pepper and mixed herbs. Cover with another layer of béchamel and repeat. For the last layer leave the potatoes without the onion and top with a final layer of béchamel.
Bake for 30/40 minutes until the potatoes are soft. If the top starts to burn and the potatoes are not ready, cover it with foil.