Clean and wash the broccoli. Cut into florets. Peel and chop the stems.
Bring 500 ml of water to a boil over high heat. Stir in the vegetable stock. Add the broccoli, cover, and cook for 5 minutes.
Remove the broccoli and set aside. Reserve the broth.
Dice the onion. Melt the butter in a large pan over medium heat. Add the onion and sauté for 2–3 minutes until softened.
Sprinkle the flour over the onion, stir well, and cook for 1 minute.
Pour in the reserved broth and whipped cream, stirring constantly. Bring to a boil over medium-high heat, then reduce to medium and simmer for 5 minutes.
Cut the smoked salmon into strips and add to the sauce.
Fold the cooked broccoli into the sauce. Season with salt, white pepper, and lemon juice to taste.