Pre-heat the oven to 180 degrees.
Place the sweet potato and onion in a large baking tray, sprinkle with some olive oil, salt ∧ pepper, mix well and bake for 20 minutes. In the meantime make the bechamel sauce by placing the milk in a pan on a low heat, add the flour, olive oil and whisk constantly until it gets ticker. Then add the salt, pepper, nutmeg and mustard and continue to whisk. Take of the heat, add the nutritional yeast and whisk again.
Assembly = 1 layer of bechamel sauce, sweet potato overlapping, spinach, onion, bechamel then repeat. The top layer is bechamel sauce and with a healthy sprinkle of black pepper. Before serving garnish with coriander.
Bake for 25 minutes