Place the chicken thighs in the bottom of the slow cooker. Top with onion, garlic, celery, and coleslaw mix.
Next, combine the soy sauce, stevia, white pepper, ginger, mustard powder, and red pepper flakes in a small bowl. Pour this mixture over the chicken and vegetables in the crockpot.
Slice the spaghetti squash in half and use a spoon to remove the seeds.
Place the spaghetti squash, cut side down, in the crockpot on top the chicken and vegetables.
Cover and cook on low for 6 hours or high for 3 hours. When the cooking time is complete, carefully remove the spaghetti squash using a towel or potholder. Use a fork to remove the squash from the skin.
Return the strands of spaghetti squash to the crockpot. Stir to combine and to break up the chicken.
Serve immediately topped with chopped green onions if desired. Store leftovers in the fridge for 5-7 days.