Place chicken in the bottom of the slow cooker.
Top with onions, garlic, spices, lemon juice, and 3/4 cup water.
Cook on high for 4 hours (of medium for 6 hours, low for 8 hours).
In a small sauce pan, whisk the cornstarch in with 1/4 cup water and the half and half.
Heat on medium, whisking often.
Once it's thick and bubbly, add it to the liquid in the slow cooker and mix in well.
Close the lid and let it cook for an additional 30 minutes.
Serve over rice or pasta.