On a baking pan, set the squash cut side up. If your spaghetti squash does not sit level, make a little stand with foil by crunching it up into an oval bowl shape formed to the bottom of your spaghetti squash.
Divide garlic, onion, black pepper, salt and shredded chicken between both empty cavities of the spaghetti squash.
Bake for 35 to 40 minutes, until the squash is fork tender.
Remove squash from oven and, using a fork, pull spaghetti squash away from the edges of the squash to easily shred it.
Top both halves with cheese and bake for another 10 minutes or until cheese is melted and slightly toasted.
Top with chives (if desired) and serve immediately.