Start by slicing each chicken breast in half lengthwise if they're on the thicker side. This helps them cook faster and more evenly. Season both sides generously with salt and pepper. I always season a few minutes before cooking so the flavor really sinks in.
Heat a skillet over medium-high heat and add a bit of butter or oil. Once it's hot, place the chicken in and let it sear for about 3 to 4 minutes per side. During the last two minutes, sprinkle the brown sugar over the chicken so it can caramelize slightly.
As soon as the chicken comes off the heat, lay the brie slices right on top while it's still hot. The residual heat is enough to melt the cheese into a gooey, creamy layer without overcooking it. I usually cover the pan loosely for a minute to speed this up.
While the chicken rests, slice your buns in half and toast them lightly in the same skillet with a touch of butter. This adds a nice golden crunch and keeps the bread from turning soggy once you add the toppings.
Spread a thin layer of mayo on the toasted buns, then layer on the chicken with melted brie, followed by tomato, red onion, and watercress. Press the top bun down gently and serve immediately while everything is warm and melty.