article on the subject of family names as trademarks so check that out, first. Assuming the chain has been using the name longer than you have, the wisest course is for you to hire a business lawyer to determine your response.
P.S. Speaking of restaurants, here's an adventurous desert we recently ordered at our local seafood shack -- Chocolate ganache wrapped in filo topped off with peanut butter sauce and hot fudge. It was great!