Sauté aromatics (optional but recommended): Heat oil in a skillet over medium heat. Add onion and cook until softened, about 5 minutes. Add garlic and ginger, cook for 1 minute until fragrant. Add curry powder, cumin, coriander, turmeric, and cayenne. Stir for 30 seconds until spices are toasted.
Add to slow cooker: Transfer the onion-spice mixture to the slow cooker (or add raw onions, garlic, ginger, and spices directly if skipping step 1).
Add vegetables: Add sweet potatoes, cauliflower, bell pepper, and chickpeas to the slow cooker.
Mix liquids: In a bowl, whisk together coconut milk, vegetable broth, diced tomatoes (with juice), and tomato paste. Pour over vegetables.
Season: Add salt and stir everything to combine well.
Cook: Cover and cook on LOW for 6-7 hours or HIGH for 3-4 hours, until sweet potatoes are tender.
Add peas: Stir in frozen peas during the last 15-20 minutes of cooking.
Adjust seasoning: Taste and adjust salt and spices as needed. For a thicker curry, mash some of the sweet potatoes against the side of the slow cooker.
Serve: Garnish with fresh cilantro and serve over basmati rice with naan bread on the side.