Place onions in a single layer on the bottom on the slow cooker followed by the chicken thighs. Top with Rotel, dried spices, and garlic. Cook on High for 3-4 hours or Low for 6-7 hours.
Remove thighs from slow cooker and broth. Shred with two forks or in stand-mixer with paddle attachment on low speed. Add broth to desired consistency. Return to crockpot with liquid to keep warm.
Notes
Trim chicken thighs: I recommend trimming as much visible fat as possible from the chicken thighs before cooking. I use kitchen shears to do this.