Heat oil in a 10-inch heavy skillet over moderate heat until hot but not smoking, then toast nuts, stirring, until golden, about 2 minutes. Transfer nuts with a slotted spoon to paper towels to drain, then sprinkle with salt to taste. Pour oil into a heatproof measuring cup.
Stir half of warm toasted walnuts into mascarpone along with 1/8 teaspoon each of salt and pepper.
Add oil in a slow stream to 1 1/2 tablespoons lemon juice in a bowl, whisking, then whisk in remaining 1/8 teaspoon each of salt and pepper. Cool dressing.
Halve pear lengthwise and remove core (preferably with a melon-ball cutter), then cut lengthwise into 1/4-inch-thick slices. Arrange slices on 4 plates, to one side, and drizzle with remaining 1/2 tablespoon lemon juice.
Toss arugula with enough dressing to coat and mound alongside pear. Spoon a dollop of mascarpone mixture onto each plate between salad and pear, then sprinkle salads with remaining toasted walnuts.