I start by slicing my baguette into half-inch rounds and laying them out in a single layer on a baking sheet. I try to keep the slices fairly even so they all bake at the same pace. Then I give them a light drizzle of olive oil across the top.
Next, I scoop a small dab of softened brie onto each baguette slice. I've found that room-temperature brie spreads far more easily than cold brie straight from the fridge. A little goes a long way here, so I don't overload any single piece.
I cut my smoked salmon into portions that roughly match the size of my baguette rounds, then lay one piece on top of each brie-topped slice. I like to fold the salmon slightly rather than laying it flat, since it gives the stacker a bit more height and texture.
I slide the tray into an oven preheated to 375°F and let everything bake for 8 to 10 minutes. I'm watching for the brie to turn soft and slightly melted and the edges of the baguette to pick up a light golden color.
Once the stackers come out of the oven, I brush each piece lightly with Dijon mustard while everything is still warm. A sprinkle of fresh cracked pepper on top finishes the look and adds one more layer of flavor before serving.