Clean and wash broccoli and cut into small florets. Dice the stems. Bring the broth to the boil, cover the broccoli and simmer for 5 minutes.
Peel and dice the onion. Cut salmon into strips. Pour broccoli on a sieve and collect the broth. Heat the butter in a saucepan and sauté the onions in it. Dust with flour, sauté briefly and pour in the broth and cream while stirring. Bring the sauce to the boil and simmer for 4-5 minutes.
Put the salmon strips in the hot sauce and let them steep for about 1 minute (better 2). Season with salt (very sparingly, because of salmon), pepper and lemon juice. Add the broccoli and briefly heat in the sauce.
Put the salmon cream in a baking dish and sprinkle the butter on top. Bake the gratin in a preheated oven (electric stove 225 ° C / convection 200 ° C / gas mark 4) for about 8 minutes. Serve with baguette.