Prep the aromatics. Chop the onion, mince the garlic, and grate the ginger.
Cut the chicken into bite-size pieces and season lightly with salt and pepper.
Bloom the spices (optional but recommended). In a skillet over medium heat, warm the oil and sauté the onion for 3–4 minutes until soft. Add garlic and ginger; cook 1 minute. Stir in curry powder, cumin, coriander, turmeric, and chili flakes; cook 30–60 seconds until fragrant.
Transfer everything to the slow cooker. If skipping this step, add the aromatics and spices directly to the slow cooker.
Build the sauce. Add crushed tomatoes, coconut milk, chicken broth, and tomato paste to the slow cooker. Whisk or stir to combine.
Add brown sugar if using.
Add the chicken and veg. Add chicken pieces, potatoes, and bell pepper. Stir to coat evenly in the sauce.
Cook low and slow. Cover and cook on Low for 6–7 hours or on High for 3–4 hours, until the chicken is tender and the potatoes are soft.
Finish and adjust. Stir in the frozen peas and cook for 10 minutes more. Add lemon or lime juice.
Taste and adjust salt, pepper, and heat.
Serve. Spoon over steamed basmati rice or serve with naan. Garnish with fresh cilantro.