Start by slicing your brie into thin, even pieces so it melts quickly and evenly in the skillet. Next, slice the avocado and toss it gently with lemon juice to keep it fresh and vibrant. Having everything sliced and ready before you turn on the stove makes the whole process move much faster.
Take your softened butter and spread it generously on one side of each bread slice. This is the side that will face outward in the skillet, so don't skip it, since it's what creates that golden, crispy crust. Make sure the butter reaches all the way to the edges for even browning.
On the unbuttered side of half the bread slices, layer a few pieces of brie followed by your seasoned avocado slices. Sprinkle a touch of salt and pepper over the avocado before topping it with the remaining bread, buttered side facing up. Press down gently so everything holds together as you move it to the pan.
Place the sandwiches butter-side down in a preheated skillet over medium heat. Let them cook for about three minutes, checking occasionally so the bread turns golden brown rather than burnt. Cover the pan loosely with a lid to help the brie melt through faster.
Carefully flip each sandwich using your spatula and let the other side cook for another two to three minutes. You'll know it's ready when both sides are deeply golden and the cheese is oozing slightly at the edges. Once done, transfer to a cutting board and let it rest for a minute before slicing.