Pierce the whole spaghetti squash 8 to 10 times all over with a sharp knife or fork so steam can escape while it cooks.

Set the steam rack inside the Instant Pot and pour in the water. Place the squash on the rack, close and seal the lid, and cook on High Pressure for 15 minutes (12 minutes for a small squash, 18 to 20 for a large one).

Quick release the pressure. Carefully lift the squash out (it will be very hot) and let it cool for about 5 minutes, then slice it in half lengthwise and scoop out the seeds.

Preheat the broiler to high and set a rack about 6 inches below the heat. Use a fork to scrape the squash flesh into strands and put them in a large bowl, leaving a thin layer in the shells so they keep their shape. Set the empty shells on a baking sheet.

In a small bowl, stir together the olive oil, Italian seasoning, onion powder, salt and pepper. Pour it over the squash strands, add the Parmesan and half of the Italian blend cheese, and toss until everything is well coated.

Pile the cheesy squash back into the shells and sprinkle the remaining Italian blend cheese over the top.

Broil for 3 to 5 minutes, until the cheese is melted and golden in spots. Keep a close eye on it, since broilers vary. Serve hot, scooping straight from the shells.