Bring a large pot of about 1,5 l salted water to a boil. Reduce the heat, and cook the asparagus in the water over low heat with the lid off for about 10-15 minutes (or until the asparagus is tender, might take less time, check in between with a fork or knife).
Drain the asparagus well, but keep the cooking water. I had 1 l of cooking water left, but you'll need only 500 ml for this recipe. Let the cooking water cool until lukewarm.
For the gravy, mix the oil with the rice flour and 3-4 tablespoons of the asparagus water in a pot using a whisk.
Heat the pan and slowly whisk in the remaining asparagus water. Keep whisking until it starts thickening and has a thick gravy consistency.
Turn the heat off and mix in the remaining gravy ingredients. Taste test and adjust with salt, pepper or nutmeg if necessary.
Serve the asparagus with the gravy, topped with parsley leaves (if you're using any).
Notes
Don't throw away the remaining cooking water from the asparagus; use it instead for other dishes, e.g. for an asparagus soup. Alternatively, you can make the double batch of the gravy and use it for other cooked or grilled vegetables, such as zucchini, carrots or potatoes. You can turn the gravy into a sauce hollandaise by adding lemon juice, a sweetener of your choice, and 2 more tablespoons of almond butter.