Preheat the oven to 400 F. Slice the fish into 1” pieces and place at the bottom of a 1 ½ quart casserole dish. Top the fish with ¼ tsp kosher salt, ⅛ tsp pepper and chopped parsley. Set this aside.
Prepare the vegetables for the saute pan. Trim the bottom of the asparagus and cut the spears into 1” pieces. Dice the carrot into small pieces. Slice the flesh off the roma tomatoes and discard the seeds. Dice the tomatoes into a small dice. Chop the red onion into a small dice. Place a large saute pan over medium high heat. Add the avocado oil and swirl the pan. Add the onions and carrots and cook for 1 minute stirring occasionally. Add the asparagus and tomatoes and cook for another minute or so. Season with ¼ tsp kosher salt and stir.
Add 1 cup of heavy cream and the dijon mustard. Stir well and allow to simmer for 1-2 minutes. Sprinkle over the xanthan gum and stir until thickened.
Pour the cream sauce over the fish in the casserole dish. Spread it evenly over the fish and poke it slightly to allow the cream sauce to run around the fish.
Bring a large pot of water to a boil and add the cauliflower florets. Cook until softened about 3-4 minutes. Drain well and place in a medium sized bowl with the remaining cream and salt.
Puree with a stick blender or place in a blender until creamy.
Top the casserole with creamed cauliflower and bake for 30 minutes or until the casserole is bubbling and the top is nicely browned. Allow to cool slightly before serving.