“À la Matelote” means “À la façon des matelots”, which is say, “in the style of sailors.”
A matelote is fish dish, either freshwater or sea, but usually freshwater, that is chopped and cooked in red or white wine. The fish used can be mixed, and can include eel. The cooking wine is then turned into a sauce with onions or shallots or both, and sometimes mushrooms in it, and sometimes thickened with a roux.
“Sauce matelote” is a wine and onion sauce.
In Normandy, cider or perry may be used instead of wine. Additionally, in Normandy, the term is sometimes applied to a dish cooked in a cauldron or casserole, in reference to how the Norman sailors would cook one-pot meals.
In classical French cooking, the term indicates a side garnish of glazed onions, mushrooms along with croutons.