Score the spaghetti squash lengthwise where you plan to cut it in half, cutting about 1/8–1/4 inch deep.
Pierce the squash in several places with a knife or fork to allow steam to escape.
Place the squash in a microwave-safe baking dish and microwave on high for 5 minutes. Do not microwave for longer than 5 minutes at this stage.
Carefully remove the squash from the microwave. Using oven mitts, cut it in half lengthwise along the scored line. Scoop out and discard the seeds.
Place the squash halves cut-side down in the baking dish. Add about 1 inch of water to the dish.
Microwave on high for 5 minutes. Check for doneness by piercing the skin with a fork. If needed, microwave for an additional 2–5 minutes, until tender.
Carefully remove the squash from the baking dish. Use a fork to scrape the flesh into spaghetti-like strands.
Transfer the squash to a serving bowl and toss with the butter, salt, and pepper. Serve warm.