Add the onion to the bottom of the Crockpot.
Place the cornish hens in the Crockpot.
Sprinkle the cornish hens on both sides with the garlic powder, onion powder, paprika, ground thyme and salt and pepper to taste.
Add the butter pieces all over and around the chicken.
Top the chicken with the sprig of rosemary.
Pour the chicken broth in the bottom of the Crockpot.
Cook on high for 4 hours.
Remove the chicken from the Crockpot, and place under the broiler for a few minutes to crisp up and brown before serving.