Remove the large pieces of pumpkin pulp from the seeds with your hands. Once most of the flesh has been removed, put the pumpkin seeds in a bowl of water and let them soak for a few minutes, allowing the tougher pieces of pumpkin to loosen. Run your hands over the seeds to get the remaining pulp off. If the seeds still don't seem clean enough, get a fresh bowl of water and repeat this process once more.
Drain the seeds and turn them out onto a clean towel, rubbing them dry. This prevents them from steaming in the oven and also removes any last remnants of pumpkin flesh.
Drizzle the seeds with the olive oil, then sprinkle with the salt and pepper and toss to coat.
Place on a sheet pan covered in tin foil, then roast in the preheated oven for 40-50 minutes, tossing every 15 minutes to ensure even roasting.