In a pot melt the butter or ghee. Leave the pot on a very little heat. Whisk in the flour. Be careful that no nubs are forming. Add slowly the milk and whisk heartily, it must be always a very smooth, even sauce. As you add the milk and whisk you can see the consistence and add more milk if you want the sauce thinner. Make it a thin cream, that is perfect.
Take away from the heat, add salt and pepper to taste and then stir in the chopped parsley.
You can change the flour to any other. I used a gluten-free flour mix, but you can also use rice flour e.g. Use a very fine one in order to get a smooth béchamel.
Also the milk can be chosen by your preference. Don't take a very tasteful one, though, as you want the taste only or the parsley.
Of course you don't need to use parsley. You can change to any other herb. Basil would be great as well. Or whatever you have in your garden growing. Mix different herbs together. Chop them always very finely.