Preheat oven to broil. Combine shrimp, shallots, tarragon, mustard, lemon juice, salt, pepper, and egg white in a bowl, and mix gently.
Lay bread slices on a baking sheet and broil about 6 in. from heat until golden brown, about 3 minutes. Turn slices over, spread with cream cheese, and top with shrimp mixture. Broil just until shrimp mixture turns pink and begins to brown, about 6 minutes.
Serve hot, garnished with tarragon sprigs and lemon wedges.