This is the ultimate hot ham and cheese sandwich. Dijon mustard and butter are spread between slices of baguette, stuffed with French ham and brie cheese, then baked for a gooey, decadent pull-apart sandwich in just 40 minutes.
Preheat the oven to 425˚ F. Line a baking sheet with parchment paper. Combine the butter and mustard in a bowl and mash with a spoon; season with salt and pepper.
Trim the ends off the baguette. Cut the bread on an angle into 12 slices, leaving the bottom of the loaf intact. (You can position chopsticks along the long sides of the bread to keep from cutting all the way through the loaf.) Transfer to the baking sheet.
Brush all but 2 tablespoons of the mustard butter between the bread slices. Fold each piece of ham in half or into quarters, then stuff into every other slice in the bread, starting with the first slice. Stuff a slice of brie into each piece of ham. Brush or rub the top and sides of the bread with the remaining mustard butter.
Bake until the edges of the ham are crisp and the cheese has melted, about 18 minutes. Pull apart or slice into individual sandwiches. Sprinkle with the cornichons and chives.