Preheat oven to 400° F. Using the tip of a thin knife or skewer, poke several holes all around each squash. And cut the top off each squash, evenly on the horizontal. Place each squash top side down on a baking tray, and roast for about an hour, or until the squash is tender to the touch.
While the squash is roasting, make the buckwheat stuffing. In a medium sauce pan, melt the oil and add the celery and onion, cooking until they start to become tender. Add the buckwheat groats and the vegetable broth. Return to boil, and reduce to simmer. Cook, uncovered until the groats are cooked and the liquid is evaporated, about 10-15 minutes.
Immediately add the pears, pistachios, thyme, and half the pomegranate seeds. Stir in salt and pepper to taste and set aside.
When the squash is ready, remove from the oven and let cool slightly. Scrape away the seeds and stringy center from the sides of the squash, while keeping the good flesh intact, and remove with a spoon, creating a cavity where you can fill with the stuffing. If there are any pieces of good squash that came off, cut them up and mix them in with the stuffing. Evenly distribute the stuffing and fill each of the squash cavities until it's overfilled and unable to take any more stuffing.
Return to the oven for 10 more minutes. Remove, transfer to a plate, and garnish with the remaining pomegranate seeds. Serve immediately