Spread half of the brie on 1 tortilla and top with half of the scallions. In a large nonstick skillet, cook the tortilla cheese side up over medium heat until the cheese is melted, 2 to 3 minutes. Top with half of the salsa and half of the walnuts; season with salt. Fold in half and transfer to a cutting board. Repeat with the remaining ingredients, then cut each quesadilla into wedges.