Prep the oven and casserole. Preheat your oven to 400°F (200°C). Grease a baking dish with a capacity of 1 to 1.5 liters using butter or oil.
Prepare the vegetables. Fill a pot with water and bring it to a boil. Add the cauliflower florets and simmer on medium-low heat for 4-5 minutes. Then, drain the cauliflower and set it aside.
Make the roux. In a skillet, melt butter and add flour. Whisk them together. Gradually stir in the milk and heavy cream in batches, creating a roux while whisking.
Ensure that the mixture becomes smooth and free of lumps. Season the roux with salt, black pepper, and nutmeg. Mix until well combined, and then add lemon juice. Whisk the roux until it thickens.
Make the cheese sauce. Remove the roux from the heat and stir in Parmesan cheese and dill. Continue stirring until the cheese is fully incorporated into the sauce.
Assemble. Pour ¼ cup of the sauce into the base of the dish. Arrange the cauliflower florets and salmon cubes in the dish. Pour the remaining sauce over the cauliflower and salmon.
Bake. Bake in the preheated oven for 30 minutes. If the top begins to brown too much, cover the dish with foil. Once done, serve and enjoy your meal.
