Preheat the oven to 400 degrees.
Heat a heavy, large skillet over medium heat. Add the seeds and shake the pan constantly or stir the seeds to prevent burning. (Seeds are toasted when they look slightly golden and are aromatic.)
Place the acorn squash halves, cut side up, in a large baking dish or on a baking sheet. Coat with cooking spray. Season with the salt, pepper, and cumin.
Roast the squash until tender, about 30 minutes. Transfer to serving plates and sprinkle each half with about 2 teaspoons pumpkin seeds.