In a skillet, gently heat the spices for 1 minute or until lightly browned. Transfer to a large bowl. Add the remaining ingredients & stir to combine.
Skewers
Add the chicken to the marinade & coat well. Thread the chicken onto 12 skewers & place in a dish. Cover & refrigerate for 1 hour. Preheat the grill, setting the burners on high. Oil the grate.
Avocado Puree
In a small food processor, purée the avocado with the sour cream & lime juice until smooth. Season with salt & pepper. Set aside in a bowl & cover with plastic wrap directly on the surface.
Grill the skewers for 5 minutes on each side or until the chicken is no longer pink.
Serving
In the center of each serving plate, spread a dollop of the avocado purée. Top with some caramelized red onions & roasted tomatoes. Finish with 2 chicken skewers.