Preheat oven to 400°.
Arrange squash in a single layer on a jelly-roll pan coated with cooking spray. Coat squash with cooking spray; sprinkle evenly with 1/4 teaspoon salt and 1/4 teaspoon pepper. Bake at 400° for 30 minutes or until squash is tender and lightly browned, stirring after 15 minutes. Remove from heat; keep warm.
Combine remaining 1/4 teaspoon salt, remaining 1/4 teaspoon pepper, vinegar, and mustard.
Cook bacon in a small nonstick skillet over medium-high heat until crisp. Remove bacon from pan, reserving 1 teaspoon drippings in pan. Crumble bacon; set aside. Add shallot to drippings in pan; sauté 1 minute. Add shallot and bacon to vinegar mixture, stirring with a whisk.
Place salad greens in a large bowl. Drizzle vinegar mixture over greens; toss gently to coat. Arrange about 1 1/3 cups salad on each of 6 plates. Top each serving with 2/3 cup squash and 1 1/2 teaspoons pumpkinseed kernels.