In the case of washed broccoli, cut the florets from the stems, halve or quarter the stalks lengthways. Blanch the vegetables in boiling salted water for 5-6 minutes, then drain.
Cut the peeled onion into fine cubes, fry in the fat but do not let it brown.
Add the flour, stir in the milk, bring to the boil, stirring constantly, and simmer for 5 minutes. Stir in the Emmentaler and season to taste.
Put the vegetables in a baking dish and pour the sauce over them. Scatter parmesan on top, put flakes of butter on top and bake for 20-25 minutes at approx. 180 degrees.