You have to order it specially because it isn’t on the dinner menu, but the cheeseburger may just be the thing at this Harvard Square institution (brunch excluded). The meat is so fresh and lean, it makes you wonder if it’s really ground filet mignon. Luckily, there’s just enough fat to provide real flavor. This burger practically melts in your mouth. It’s served on a wide, flavorful bun, with cheddar and fresh lettuce, tomato, and red onion, and a side of outstanding, crispy fries. The waitstaff may be a tad buttoned up, but that’s forgiven with the first juicy bite.