Preheat the oven to 350°F (175°C) and lightly grease a 9x13 inch baking dish.

Melt the 3 oz (85g) of butter in a large skillet over medium-high heat, add the 15 oz (425g) of sliced mushrooms, and fry for 5 minutes until golden brown.

Stir the 0. 1 oz (3g) of chopped parsley, 1 tsp (5g) of salt, and 0. 5 tsp (1g / 0.04 oz) of black pepper into the browned mushrooms.

Reduce the heat to low, pour in the 2 cups (473ml) of heavy whipping cream, add the 2 tbsp (30g / 1.06 oz) of Dijon mustard, and allow the sauce to simmer gently for 10 minutes until slightly thickened.

Season the 25 oz (708g) of white fish slices lightly with salt and pepper, then arrange them in an even layer at the bottom of your prepared baking dish.

Sprinkle 6 oz (170g) of the shredded cheese evenly over the raw fish slices.

Pour the creamy mushroom and mustard sauce completely over the fish and cheese layer.

Top the casserole evenly with the remaining 2 oz (55g) of shredded cheese and bake in the preheated oven for 30 to 35 minutes until the fish flakes easily and the cheese is bubbly and golden.

While the casserole is baking, bring a large pot of salted water to a rolling boil, add the 20 oz (567g) of cauliflower florets, and boil for 5 minutes until completely tender.

Drain the cauliflower florets thoroughly to remove excess moisture and transfer them to a large mixing bowl.

Pour the 3 oz (85ml) of olive oil into the strained florets, mash thoroughly until smooth and creamy, season with salt and pepper to taste, and mix well before serving alongside the hot fish casserole.

Pat the Fish Dry: White fish holds a lot of water. To prevent your creamy Dijon sauce from splitting or becoming too thin, thoroughly pat your fish fillets dry with paper towels before seasoning them.

Squeeze the Cauliflower: Cauliflower absorbs water like a sponge when boiled. For the creamiest, least watery mash, drain the florets well and gently squeeze out excess moisture using a clean kitchen towel before blending them with the olive oil.