Optional flavour step: Lightly sauté onion, garlic, and ginger for 3–5 minutes.
Add to slow cooker: Add potatoes, onion mixture (or raw onion), garlic, ginger, tomatoes, coconut milk, spices, salt, and pepper. Stir well.
Cook: Low: 6–8 hours; High: 3–4 hours.
Add peas: Stir in frozen peas in the last 20–30 minutes.
Finish: Stir in garam masala just before serving and adjust seasoning.