Preheat oven to 450F.

Heat oil in a large nonstick skillet over medium-high heat.

Add onion and cook, stirring often, until soft and golden, about 7 minutes.

Stir in pear and cook, stirring often, until slightly soft and heated through, 1 to 2 minutes.

Add balsamic vinegar and pepper and continue cooking, stirring often, until the liquid has evaporated and the onion is tender and coated with a dark glaze, about 2 minutes more.

Divide the onion-pear mixture among the pitas.

Sprinkle with cheese and walnuts.

Transfer the pitas to a large baking sheet.

Bake until crispy and the cheese is melted, 10 to 15 minutes.