In a slow cooker, combine the tomato sauce, coconut milk, onion, garlic, tomato paste, garam masala, sugar, salt (I use about 3/4 teaspoon) and several turns of the pepper mill.
Place the chicken on top, flipping to moisten with the sauce. I put a few of the onion slices on top of the chicken as well.
Cover and cook until the chicken is tender, 3-4 hours on high or 6-7 hours on low.
Remove the chicken to a plate, and shred the meat with two dinner forks. Stir the shredded meat back into the sauce and rewarm for a few more minutes.
Serve over the cooked rice and garnish with cucumbers and fresh herbs, as desired.