Preheat the oven to 450 degrees . In a large, ovenproof skillet, heat 3 tablespoons olive oil over high heat. Add the potatoes, stir to coat and cook, stirring frequently, until lightly browned, about 8 minutes.
Add the pears, thyme sprigs, 1 teaspoon salt and 1/2 teaspoon pepper to the potatoes; stir to combine. Transfer the skillet to the oven and roast, tossing halfway through, until the potatoes and pears are tender, 12 to 15 minutes.
Meanwhile, in a small bowl, whisk together the lemon juice, honey, mustard and the remaining 1/4 cup olive oil; season with salt and pepper and set aside. Place the watercress in a shallow bowl or on a platter and set aside.
Remove the skillet from the oven and discard the thyme sprigs. Sprinkle the lemon peel and thyme leaves over the roasted pears and potatoes. Drizzle 3 to 4 tablespoons dressing on top and toss. Season with salt and pepper.
Spoon the pears and potatoes over the watercress and drizzle the remaining dressing over the greens.