Slice each baguette in half lengthwise without cutting all the way through, so it opens like a book.

Spread the softened butter over one cut side and the Dijon mustard over the other.

Layer the ham, turkey, and brie evenly down the length of each baguette.

Scatter the arugula and cornichon slices over the meat and cheese.

Close the baguette, press down firmly along its length with your palm, and wrap it tightly in plastic wrap.

Refrigerate the wrapped baguettes for at least 1 hour, which lets the bread take the shape of the filling and makes clean slices possible.

Unwrap and slice each baguette on a sharp diagonal into pieces about 2 inches wide.

Arrange the pieces cut-side up on a long platter so the layers show.