3 cups butternut squash, peeled and cubed
1/2 teaspoon black pepper, divided
2 medium shallots, minced
1/4 cup balsamic vinegar
2 tablespoons water
2 teaspoons Dijon mustard
10 cups (about 10 ounces) mixed baby greens
2 tablespoons raw pumpkin seeds, lightly toasted (see note)