Add onion, garlic, salt and pepper to a medium saucepan. Cook over medium heat for one or two minutes, until fragrant and slightly softened.
Throw the cauliflower florets in and continue cooking for a minute or two.
Add chicken stock, cover and bring to the boil; lower heat and continue cooking until the cauliflower is tender, about 5-7 minutes.
Ladle the cauliflower mixture into your blender; add ghee, Dijon mustard and process on high speed until super smooth and silky in consistency. Set aside.
Cut the Brussel sprouts in halves or quarters, depending on size, and steam them for a few minutes over boiling salted water.
Add the cooked fish and Brussel sprouts to a large mixing bowl, followed by “Bechamel” sauce. Mix very delicately until well combined and transfer to an oven safe dish.
Drop dollops of cooked butternut squash all over top, sprinkle with chopped pistachios and drizzle a little bit of extra virgin olive oil or melted ghee.
Cover with foil and place in the oven for 30 minutes; after that time, remove the foil and set the oven to broil for a 2-3 minutes or until the top gets a slight golden coloration.
You could also make this ahead of time and place it in the fridge after you have covered it with foil (although you might want to let it cool a bit beforehand). In this case, you’ll want to increase oven time to about 45 minutes.
This also reheats extremely well in the microwave and will easily keep for 2-3 days in the refrigerator.