Toss the pears, sugar, salt, and pepper in a bowl. Heat the vegetable oil in a 12-inch nonstick skillet over medium-high heat until the oil just begins to smoke.

Add the pears to the skillet in a single layer, cut-side down, and cook until they are golden brown, 2 to 4 minutes. Using a fork, flip each pear onto the other cut side and cook until light brown, another 2 to 4 minutes.

Turn off the heat but leave the skillet on the burner. Add the vinegar and stir gently until the liquid reduces to a thick glaze that coats the pears, about 30 seconds.

Transfer the pears to a large plate and let them cool completely to room temperature, about 45 minutes, then cut each quarter crosswise into half-inch pieces.

Whisk the remaining vinegar, remaining sugar, and the shallot together in a large bowl. Whisking constantly, drizzle in the olive oil until the dressing is fully emulsified.

Add the lettuce, watercress, and pears to the bowl and toss gently to coat with the vinaigrette. Season with salt and pepper to taste and divide the mixture among salad plates.

Top each serving with pecans and shaved Parmesan. Serve immediately while the greens are still crisp.