For the dressing:
Combine vinaigrette, raspberries, and sugar in a blender. Blend until smooth; set aside.
For the salad:
In a large bowl, toss together spinach, onions, raspberries, cucumbers, and half of the vinaigrette.
Divide mixture among 4 chilled plates. Add chicken strips to mixing bowl and toss with 2 tablespoons dressing. Divide among the plates. Garnish with feta, pecans, and black pepper. Serve.