Preheat broiler. Mix mayonnaise, parmesan, green onions, dill, lemon juice, and pepper.
Set baguette slices on a baking sheet and broil about 4 in. from heat just until starting to brown, about 1 minute. Turn slices over and spread parmesan mixture generously on top. Broil until topping is bubbly and starting to brown, 1 to 2 minutes. Serve warm.