This always happens. I went in for cheese, and left with cheese, jars of fancy mustard and ketchup, and an awesome sandwich. Farmstead, Wayland Square's terrific cheese boutique that shares a wall (and a door) with chef/owners Matt and Kate Jennings' beloved bistro La Laiterie, serves lunch between 11 a.m. and 3 p.m, and the roast beef sandwich ($9) came highly recommended by the friendly on-duty cheesemonger.
The top round is roasted in house to medium-rare pinkness, then peeled into paper-thin slices before being piled on crusty Seven Stars ciabatta and lightly pressed on the griddle. Also between the bread: ripe, dense, creamy bleu cheese crumbles; thin-shaved pickled red onion; greens so fresh that they almost crunch; and a smear of aioli that stands out surprisingly well against all those bolder-flavored condiments. I should also mention that the lard-fried potato chips (which taste remarkably like pâté) originally came on the side, but I crammed a few of them onto the bread as well; to me, most sandwiches are improved with chips.