Cut the broccoli into florets.
Cook in salted water until tender.
Rinse under cold running water and drain well.

Peel the onion and cut it into thin slices.
Melt the butter in a small saucepan, add the sliced onion and cook over low heat until softened, without browning.
Put the drained broccoli, cooked onion, and ricotta in a food processor.
Reduce to a smooth puree. Add the wasabi and lemon juice.
Mix well, then check the seasoning.
Cover and refrigerate until ready to serve.