Combine butter, onion, carrots, garlic, curry paste, ginger, garam masala, brown sugar, cumin, tomato paste and salt in a medium microwave-safe bowl. Cover with a paper towel and microwave on high power for 3 minutes. Stir well and cook for another 3 minutes, again covered with a paper towel.
While the onion mixture is cooking, trim as much of the fat off of the chicken thighs as possible and cut into slightly larger than bite size pieces (chicken will shrink as it cooks).
Combine onion mixture, coconut milk and broth in a slow cooker with temperature set to low.
Add chicken thighs and stir to combine. Cover and cook for 4/1/2 to 6 hours or until carrots are super tender.
Increase slow cooker temperature to high. Combine water and cornstarch and stir well until smooth. Add corn starch mixture to slow cooker and cook for another 10-15 minutes on high or until sauce thickens a bit.
Taste and add more salt and/or garam masala, if desired.
Combine chopped cilantro and chopped cashews. Garnish with cilantro/cashew mixture and sliced avocados. Serve with basmati or jasmine rice and/or warm naan or flatbread.