Preheat oven to 300 degrees. Line a baking sheet with SILPAT.
Add brown sugar to a plastic bag and set aside. Whisk egg white until frothy, then mix with pecan halves. Add to plastic bag and toss to coat evenly. Arrange in single layer on lined baking sheet and bake for 40 minutes, flipping halfway. Set aside to cool.
Add all dressing ingredients except olive oil and poppyseeds to a wide mouth jar. Blend with an immersion blender while slowly pouring in the olive oil. Stir in poppyseeds and set aside.
Season chicken with Lemon Pepper Seasoning, then grill until cooked through, about 5 minutes per side (depending on thickness). Allow chicken to rest for 5 minutes before slicing.
Divide arugula, berries, onion, chicken, glazed pecans, and goat cheese (if desired) into 4 serving dishes. Top with Lemon Poppyseed Dressing before serving.