Clean the French beans and cook in salted water until al dente. Let cool down lukewarm. Clean the chanterelles, sprinkle with flour in a colander under the shower head (the dirt is bound with the flour and rinsed off). Steam the onion in the butter until golden. Add the chanterelles, turn them and cook, season with a little salt and pepper, let them cool down lukewarm.
Mix the lukewarm beans and mushrooms. Baste with a sauce made from balsamic vinegar, pepper, salt and oil.