As soon as the leek and onions are soft sprinkle them with the flour, stir well and cook for a couple of minutes. Gradually add the warmed milk stirring constantly. Cook the mixture for a bunch of more minutes until it thickens, then add the nutritional yeast (or vegan cheese previously grated or finely chopped). Stir until the cheese has melted, then add the fresh spinach and give it a last stir. Set aside.