The Galette-Saucisse Moutarde is the Breton grilled-sausage-in-a-galette with mustard, and the mustard is treated less as an option than as the standard accompaniment. A pork sausage is grilled until the skin blisters, a buckwheat galette is cooked thin on a hot iron, a stripe of mustard is laid down, and the sausage is rolled inside the galette lengthwise so the bread closes around it as a sleeve. There is no plate and no fork. The mustard is applied inside the wrap before it closes, so it coats the length of the sausage rather than sitting as a dip on the side.
That placement is the whole point of the craft. A grilled pork sausage is fatty and a little charred, the buckwheat galette is nutty and faintly bitter, and the mustard is the sharp third element that cuts both: a line of it down the sausage hits in every bite rather than only where you dunk. The sandwich is built to be eaten on foot, rolled tight so it holds together in the hand, bought from a stall and walked with, which is why the condiment goes inside the construction instead of on a plate beside it. It is eaten straight off the grill, while the galette is still pliable and the sausage hot, and like its plain sibling it is a fixture of Breton markets, fairs, and the stands outside stadiums.
This is a Brittany street-food sandwich, and it is the codified variation of the plain Galette-Saucisse: a single common addition that recurs often enough to have earned its own name, which is the same impulse that turns any small swap into a separate sandwich. The wider tradition keeps the buckwheat wrapper constant and changes what it holds: ham and cheese and egg in the folded complète, smoked andouille in another corner of the region, the sweeter wheat-flour crêpe for dessert. Those relatives are gathered under Crêpe & Galette Salée, and the Galette-Saucisse Moutarde is the hand-held member with the mustard already in it.