Place the onions in the bottom of the slow cooker.
Top with chicken.
Top the chicken with all other ingredients except the half and half and corn starch.
Cut the lime in half and juice one of the halves into the slow cooker. It should be about 1 Tbsp of lime juice.
Cook on high for 4 hours (or medium for 6, low for 8)
Remove chicken from the slow cooker and cover to keep warm.
In a small sauce pan, whisk together the corn starch and half and half.
Heat over medium heat, whisking often until it bubbles and thickens, and then add in the butter and whisk it in as it melts.
Add the thickened half and half to the liquid in the slow cooker and blend well.
Close the lid and let the sauce cook for 5 minutes.
Serve chicken. Top with sauce and garnish with additional cilantro, lime wedges from the other half of a lime, and lime zest if you like.