In a small bowl, mix together the ingredients for the spice blend. Set aside.
Melt 2 tbsp of the butter in a large frying pan over high heat. Thoroughly coat the chicken on all sides with ½ the spice blend. Working in batches so that the chicken pieces don’t touch each other in the pan, add a portion of the seasoned chicken to the pan and sear for 2 to 3 minutes on each side, just until the chicken develops a light brown crust. Transfer the browned chicken to the slow cooker and repeat.
Once the chicken is finished, melt the remaining 1 tbsp of butter in the pan. Then add the rest of the spice blend and chopped onion. Stir for 8 to 10 minutes, until browned. Whisk in the tomato paste and cook for another 4 to 5 minutes, until the paste turns deep red. Stir in the crushed tomatoes, coconut milk, coconut cream, and lime juice and bring to a simmer. Let simmer for 10 minutes, or until the sauce has thickened slightly and coats the back of a spoon. Pour the sauce over the chicken in the slow cooker, and stir so that the chicken is coated on all sides.
Cover and cook on high for 4 hours or on low for 8 hours. Garnish with the jalapeño slices and fresh cilantro.