Cut squash in half, de-seed and remove visible strings (like a pumpkin).
Place in pan and cook for 1 hour (may vary depending on size of squash) until soft.
While squash is cooking, combine beef, onion, eggs, salt and pepper in a big bowl.
Mix well with hands or food processor.
Roll into 1"-2" size meatballs (depending on how you like them). Be sure to pack them tightly.
Add olive oil to a pan, and heat until shimmery.
Add meatballs and sear. Don't crowd your pan- cook in about 3 batches. Dont worry about getting meatballs to cook through- you are just browning them enough to keep them firm while they finish cooking in sauce and give them a little flavor.
Once meatballs are browned, toss into pot with sauce on low, and cover, adding meatballs as you brown them.
Let cook for over 40 minutes on low. Stir occasionally to prevent burning.
When squash is soft, remove from oven and let cool until you can handle it without burning your hands.
Run a fork across squash (like a rake) to get the squash to form "spaghetti" like strands, removing all the flesh off the rind.
Add spaghetti and meatballs, and serve in squash rind or a big bowl.
Top with any garnish you'd like (I love parmesan- its not really paleo, but a little goes a long way, so I just go with it).
Serve warm!