Thaw the cauliflower in a water bath and place in a baking dish. Make a roux from the butter and flour. Deglaze with the broth, the juice of half a lemon and the milk. Stir well so that no lumps form. Add dill to taste and let the sauce simmer.
Chop the salmon and pour over the cauliflower. Season the sauce with salt and pepper and then distribute it on the casserole. Sprinkle with cheese and grill in the oven at 180 degrees.