1. Make squash: Preheat oven to 400°F; line a large baking sheet with foil and mist with cooking spray. Toss squash with oil and lemon juice in a bowl. Arrange in a flat layer on a baking sheet. Season with salt and pepper. Roast until lightly browned, about 20 minutes. Turn each cube over, season with salt and pepper again and cook until browned and slightly caramelized around edges, about 20 minutes longer. Let cool on baking sheet. (Can be made up to 3 days in advance. Layer on paper towels on a plate. Wrap plate in plastic wrap and refrigerate. Bring to room temperature before using.)
2. Make seeds: Preheat oven to 350°F. Combine seeds, butter, sugar, cinnamon, cumin, paprika and salt in a bowl. Spread over a baking sheet. Toast, stirring often, until crisp, about 15 minutes. Transfer to a bowl and cool completely. (Can be made up to 3 days in advance. Keep covered in a dry, cool spot.)
3. Make dressing: Whisk vinegar, salt, pepper and mustard in a large bowl until blended. Slowly add both oils, whisking constantly until blended and thickened.
4. Just before serving, pile greens in bowl on top of dressing. Add squash and 3/4 cup pumpkin seeds; gently toss. Top with remaining seeds and serve.