Pour the coconut milk, tomatoes, and tomato paste into the slow cooker. Stir to combine.
Add the red pepper, onion, sweet potatoes, and garlic followed by the curry powder, sugar, salt, pepper, and red pepper flakes. Stir to combine.
Place the chicken thighs on top, and flip to coat with the sauce. (I like to spread a little of the onion mixture on top of the chicken, but this isn’t critical.)
Cook on low heat for approximately 6-7 hours or on high heat for 3-3 1/2 hours. If you are using cauliflower, it is best to add that in the last 90 minutes of cooking time so that it does not overcook. Peas, if using, may be stirred in (still frozen is fine) during the final 15 minutes. The chicken may be removed to a plate and pulled into bite-size pieces with two forks or can be carefully pulled apart while still in the slow cooker.
Serve over rice and sprinkle with chopped fresh cilantro, if desired.