Slice your baguette in half lengthwise, or grab your bread slices if you're using a loaf. Lightly butter the outside of each piece so it crisps up nicely once it hits the pan. This little step makes a big difference in texture, giving you that satisfying crunch with every bite.
In a small bowl, stir together the Dijon mustard and mayonnaise. This combo adds moisture and a subtle tang that cuts through the richness of the brie. Spread it generously on the inside of both bread pieces so every bite gets that flavor boost.
Place the sliced ham on the bottom half of the bread, folding it slightly so it doesn't hang off the edges. I like to layer it in loose folds rather than flat sheets, since it gives the sandwich more texture and makes each bite feel heartier.
Lay your brie slices right on top of the ham, rind and all if you like that extra earthy flavor. The heat from grilling will melt it beautifully, so don't worry about it looking a little messy at this stage. It all comes together once it hits the pan.
Scatter the arugula or watercress over the brie, then season with a pinch of salt and pepper. The greens add a fresh, slightly bitter contrast that keeps the sandwich from feeling too heavy. Close it up with the top half of the bread.
Heat your skillet over medium heat and place the sandwich in, pressing down gently with a spatula. Grill for about 3-4 minutes per side, until the bread turns golden brown and the brie is soft and melty inside.
Remove the sandwich from the pan and let it rest for a minute before slicing. This helps the cheese set just slightly so it doesn't ooze out everywhere. Cut it in half or into smaller pieces, and it's ready to enjoy.