A 'mâchon' is a 9 AM bouchon-style breakfast: charcuterie, wine, cheese. Workers' tradition kept alive by a few places.
Café des Federations and Le Garet still serve mâchons on certain mornings. Very heavy; plan a long walk after.
Ask locals — 'mâchons' aren't on menus; they're informal communal breakfasts that need to be timed.