Scoop out the pumpkin's seeds and pulp with a spoon and put into a colander. Rinse away all the stringy stuff. Give the seeds a separate rinse to make sure they're clean.
Spread the seeds out on a rimmed baking sheet and roast until dried out, 30 to 45 minutes. Toss with enough oil to nicely coat (this will depend on how many seeds you harvested), season with salt and sprinkle with the za'atar spice. Return the seeds to the oven and continue to roast until crunchy and golden, 20 to 30 minutes more.