Preheat oven to 250°F (120°C). Cut off top 3 to 4 inches of pumpkin then scoop out seeds onto a clean work surface. Discard stringy fibre along with any seeds that are cracked, then transfer to a strainer and rinse well. In a medium pot, bring water to a boil. Add seeds, reduce heat and boil gently for 10 minutes. Drain well then transfer to a paper towel-lined sheet tray and pat dry.
Transfer seeds to a medium bowl, toss with oil and spread out in a single layer on a large baking sheet. Roast seeds, stirring every 10 minutes or so, until just crisp and golden brown, about 1 hour. (They will become crispier as they cool). Set aside to let cool completely.
For spicy pumpkin seeds, mix ½ teaspoon garlic salt, cumin, coriander and cardamom with seeds and oil before roasting. For sweet pumpkin seeds, mix 1 teaspoon each ground cinnamon, cloves and ginger and 1 ½ tablespoons dark brown sugar with seeds and oil before roasting.