Place onion in crock pot. Top with chicken bones (and skin and meat, if using), celery, and carrot. Pour water over the contents, leaving 1 inch of head space. Sprinkle the salt, Italian seasoning, red pepper flakes, and bay leaf over the water. Cover and cook on low overnight.

In the morning, turn off the heat and remove the lid. Allow to cool slightly and skim off the fat. Strain through a mesh sieve and ladle into freezer safe containers or bags. Label and freeze for up to 6 months.