Sauté the beef with onion, garlic, salt, pepper. Added crushed tomatoes, bay leaf and cheese rind, stir. Pierce the spaghetti squash all over with a knife and place over the sauce. Cook high pressure 15 minutes, or until the skin of squash easily gives when pressed with a spoon. Let the pressure release, discard bay leaf and rind and transfer the squash to a bowl to let cool.
When cool enough to handle, cut in half with a large knife, discard the seeds and scoop the strands out with a fork. Place in a colander to drain well (I use paper towels as well to soak up any excess moisture) then serve sauce over squash with grated cheese if desired.
For the slow cooker, saute meat as above in skillet, transfer to slow cooker with remaining ingredients as listed above and cook low 6 to 8 hours.