Toss the pears, sugar, salt, and pepper together in a bowl until the fruit is evenly coated.

Heat the vegetable oil in a large nonstick skillet over medium-high heat until the oil begins to shimmer and just starts to smoke.

Place the pears in the skillet in a single layer, cut-side down, and cook until they turn a deep golden brown, about 2 to 4 minutes. Use a fork to tip each pear onto its other cut side and cook until light brown, another 2 to 4 minutes.

Turn off the heat but leave the skillet on the burner. Pour in the first portion of vinegar and stir gently until it bubbles, reduces, and becomes a thick glaze that coats the pears.

Transfer the pears to a large plate and let them cool to room temperature, which helps the glaze set and the texture firm up. Once cool, cut each quarter crosswise into half-inch pieces.

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

Add the lettuce, watercress, and pears to the bowl and toss gently to coat. Season with salt and pepper to taste, then divide among plates. Finish with the pecans and Parmesan and serve immediately.