Preheat to 450 degrees F.
Cut 16 thin slices (about 1/2" each) from a baguette.
Butter one side of the 8 slices of bread and place each buttered side down on a baking sheet.
Spread about 1 1/2 tsps of jam on each slice, then top with two thin slices of Brie (about 1 oz of cheese per sandwich).
Next, spread about 1 tsp of Maille Old Style mustard on each of the remaining 8 bread slices and place them mustard-side down on the cheese to form sandwiches.
Butter the tops of the 8 sandwiches.
Bake for 4 minutes, then carefully flip the sandwiches and bake for an additional 4-5 minutes, or until the bread is golden brown and the cheese is melted.
Transfer the grilled cheeses to a platter and serve immediately.