Preheat oven to 450 degrees F with rack in the middle. To help with clean up, line a 17-by 12- by 1-inch baking sheet with foil. Gently spray with cooking spray and set aside.
Peel the squash and cut off the ends. Cut in half and remove the seeds.
Cube the squash into ½-inch cubes – about 6 cups. In a large bowl toss the squash with the 2 tablespoons olive oil, salt and cumin to taste. Arrange in single layer on prepared baking sheet.
Roast the squash, turning occasionally, until golden brown on edges, about 30-35 minutes.
Toast the pepitas in 1 tablespoon oil in a large heavy skillet over medium-high heat, stirring frequently, until seeds are puffed and beginning to brown, about 2 to 4 minutes. Transfer to a large plate to cool.
Using a food processor, pulse the cooled pepitas with the remaining 1 tablespoon olive oil, garlic, cilantro, lemon juice and salt and pepper to form a coarse paste.
Toss the squash with the pesto. Taste, adding additional salt and pepper if desired. Serve immediately.