Place oven rack in the lowest position; preheat to 450�F. Coat a large baking sheet with cooking spray.
Heat oil in a large skillet over medium-high heat. Add onions and cook, stirring occasionally, until starting to brown, about 6 minutes. Reduce heat to low, cover and cook, stirring occasionally, until very soft and golden, 5 to 8 minutes more.
Meanwhile, roll out dough on a lightly floured surface to the size of the baking sheet. Transfer to the baking sheet. Bake until puffed and lightly crisped on the bottom, 8 to 10 minutes. Toast walnuts in a small dry skillet over medium-low heat, stirring, until lightly browned and fragrant, 2 to 3 minutes.
Stir vinegar, sage and pepper into the onions. Spread on the crust and top with pears, walnuts and cheese. Bake on the bottom rack until the crust is crispy and golden and the cheese is melted, 11 to 13 minutes. Slice and serve.
The classic combination of pears, walnuts and blue cheese isn't just for salad. Toss the same ingredients onto purchased whole-wheat pizza dough and make your own gourmet flatbread at home. Any type of ripe, firm pear will work-red pears look great.
