you follow the same method as you would for a normal bechamel (white sauce) but here we just replace the dairy for oil and a nut milk. I use almond milk but I'm sure other nut milks would be great too. The amounts are very approximate here as you'll need to feed the roux and add liquid in accordingly.
4 tbsp rapeseed oil
3/4 cup flour
2-3 cups almond milk (make sure you use an unsweetened one.)
water
4 tbsp cashew cream (see recipe)
4 tbsp nutritional yeast flakes
grated nutmeg
salt and pepper
1) in a pan, add rapeseed oil and flour and, using a hand whisk, mix the two on low heat
2) once the oil and flour have combined, add a little milk to the mix to liduidise it and stir continuously to remove any lumps. Keep an eye on the heat and keep feeding the sauce with milk. Taste the sauce as you're cooking to make sure the flour has 'cooked out'. If you cook it too little, it'll just taste like grainy flour.
I like to add a little water in the end, in place of milk to stop the sauce going too thick. Once you've reached the desired thickness, stir in the cashew cream, yeast flakes, nutmeg and season.