Heat broiler. Remove seeds from an unpeeled 1 lb butternut squash half. Cut into 1/4"-thick slices and put on baking sheet lined with nonstick foil. Coat with cooking spray. Broil squash 5" from heat, turning, until golden brown, 9 min. Let cool. Whisk together 2 Tbsp sherry vinegar, 1 Tbsp olive oil, 1 Tbsp honey, and 2 tsp Dijon mustard in large bowl. Add 4 cups baby spinach, 1 cup torn radicchio, 1/4 cup feta crumbles, and 3 Tbsp toasted pumpkin seeds. Toss salad and stack between squash slices on 4 plates.