In a food processor, whirl parsley, parmesan cheese, olive oil, tarragon, garlic until finely chopped.
Rinse, peel, core, and slice pears; in a bowl, mix with lemon juice.
Slice baguette in half crosswise, then lengthwise. Arrange pieces cut side up on a 10- by 15-inch baking sheet and broil 4 to 6 inches from heat until lightly browned, 1 1/2 to 2 miuntes.
Spread tarragon pesto on toasted bread. Overlap pear slices on top. Sprinkle gruyère cheese evenly over pears, then generously grind pepper over cheese.
Return to broiler until cheese is melted, about 2 minutes. Slice diagonally into 1 1/2- to 2-inch-wide pieces.