Place 1/4-1/2 inch thick baguette slices on a sheet pan. Brush both sides of each slice lightly with olive oil and sprinkle with salt and pepper.
Bake 12-15 minutes until golden, flipping halfway through. Remove from oven and cool to room temperature.
Top each crostini with one thin slice of brie, 2 thin slices of apple, then 2-3 pieces of walnut, drizzle lightly with honey, and top with a small sprig of lemon thyme. Serve within an hour.
If you're planning to make the crostini ahead of time, you can make them up to one week in advance. Store them in a sealed container at room temperature.You can freeze the crostini in advance, too. Make sure to remove all the air from the bag and remove them from the freezer 1 hour before serving.