Meanwhile, have foil handy. In a small skillet, combine the sugar and ¼ tsp. salt; cook and stir over medium-low heat, 3 to 4 minutes, scraping sides of skillet as needed, until sugar is smooth, melted, and golden (it burns quickly, so watch it closely). Add nuts; stir to coat. Watch the color and as soon as sugar turns a rich (not dark) caramel color (several seconds later), immediately transfer the nuts to foil. Pour caramel left in pan over nuts (nuts become crunchy as they cool). When cool, break nuts apart; set aside.
Use a slotted spoon to transfer pears and onions to a large bowl. Add lettuce and just enough dressing to lightly coat; toss gently. Add the cheese and pecans; toss gently. Season with salt and pepper to taste. Serve immediately.