Heat a nonstick skillet over medium heat. Spread each tortilla with 1/3 of the mustard mixture and place in the hot skillet, mustard side up. Arrange 1/3 of the brie slices on half the tortilla. Once the cheese starts to melt, arrange 1/3 of the apple slices on the cheese and top with 1 cup arugula. Sprinkle with 1/4 teaspoon black pepper. Fold tortilla in half and press with a spatula to stick the sides together. Cook about 2 minutes or until golden brown, flip, and cook another 2 minutes on the other side. Cut into 4 wedges. Repeat with remaining ingredients.