Cut spaghetti squash in half lengthwise. Scoop out seeds and pulp. Place trivet in pot, along with 1 cup of water. Place squash on top of trivet. Lock lid and cook for 2 minutes at high pressure. Once cook time is complete, quick-release pressure. Remove squash from pot and set aside to cool.
Clean out pot, then... Using sauté setting – add oil, onions, and 1/4 teaspoon of salt to pot and cook for 3-4 minutes, until soft and translucent.
Add mushrooms, garlic, oregano, basil, and fennel seeds to pot. Cook for about 10 minutes, until mushrooms have released their juices and then evaporated. Remove and set aside.
Continuing on sauté setting – add ground beef, 1 teaspoon of salt, and black pepper to pot. Break beef up as much as possible and cook until thoroughly browned. Drain and return to pot.
Return mushrooms to pot, along with garlic powder, and onion powder and mix well. Pour in crushed tomatoes, tomato sauce, 1 teaspoon of salt, and sugar. Mix well. Then lock lid and cook for 5 minutes at high pressure.
Meanwhile... Using a fork, scrape out flesh from each half of squash, making sure not to get any shell. Place in a large bowl. Just before transferring to serving dish, season spaghetti squash with salt to taste.
Once cook time is complete, quick-release pressure. In each individual serving dish, add spaghetti squash and top with desired amount of sauce.