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 chicken is tender, 3-4 hours on high or 6-7 hours on low.
Shred chicken thighs, if desired, or leave whole. Serve chicken and sauce atop the cooked rice and garnish with cucumbers and fresh herbs, as desired.