Preheat the oven to 350 degrees . In a small bowl, toss the walnuts with the egg whites to coat. Add the brown sugar and toss again. Transfer to a rimmed nonstick baking sheet and toast in the oven for 15 minutes. Let cool, then break apart.
In a small bowl, whisk together the granulated sugar, lemon juice, onion, mustard, poppy seeds and salt. Pour in the oil in a slow, steady stream, whisking constantly.
In a serving bowl, combine the lettuce, bell pepper, scallions and dressing; toss to combine. Top with the pears and candied walnuts.