Rinse the pears but don't peel them. Cut them into small dice and place in a small bowl. Sprinkle pears with lemon juice and toss. Add the parsley, blue cheese, and oil and toss gently and thoroughly. Season to taste with freshly ground black pepper. Set aside. (The topping can be prepared up to this point up a few hours ahead. Store, covered, in the refrigerator.)
Preheat the broiler. Arrange bread slices n a single layer on a baking sheet and toast under the broiler until lightly browned and crisp, 1 to 2 minutes per side.
Spread each toast round with a teaspoon of mascarpone. Top with a generous Tbsp of the diced pear mixture, mounding it up high and pressing it firmly. Finish each one with a sprinkling of walnuts and serve.minutes.
Servings: 24