Prepare the steak of your choice. Boil the broccoli and cauliflower. You can mash them both together (but on the picture above it's only the cauliflower that has been mashed). Add the milk (or cream) and some salt and pepper, and possibly a little sweetener ( Splenda, stevia, or similar).
Chop the mushrooms, and heat them in a frying pan with butter. When done, put them aside, and use the frying pan to make sauce.
The sauce: brown some butter in a deep frying pan. Add some locust bean gum (about a teaspoon is probably enough) and stir, and then add just a little water at a time, while it boils. Add salt and pepper to give the sauce flavor.