Preheat your barbecue or indoor grill to medium-high. Quarter the pear lengthwise and remove the core. Cut the pear, lengthwise, into 20 thin wedges. Toss the pear slices with the lemon juice and 1 Tbsp. of the oil. Oil the bars of your grill. Grill the pears, in batches, about 30 seconds per side, and then set on a plate.
Set an oven rack 6-inches below your oven’s broiler. Preheat broiler to high. Set the baguette slices on a baking sheet. Lightly brush or drizzle them with the remaining 2 Tbsp. of oil. Broil the bread until lightly toasted, and then arrange on a platter.
Top each crostini with a slice of cheese and a slice of pear. Sprinkle with black pepper and drizzle with the vinegar. If desired, garnish each crostini with a small rosemary sprig and then serve.
Note: The baguette used for this recipe was about 2 1/2- to 3-inches wide.