Season the lamb with salt and pepper, add a little oil, and mix well. Heat a large heavy-based pan over a medium-high heat. Brown the lamb in 2 or 3 batches. Remove each batch and set aside.
Add the onion to the pan and cook until soft. Add the chilli and curry paste. Stir for 2 minutes and pour in the stock. Bring to the boil, and then reduce heat to simmering point.
Return the lamb to the pan, simmer for 30 minutes. Check that the dish simmers gently during the cooking time. Stir occasionally and adjust the heat if needed.
Stir in the coconut milk, sugar and fish sauce. Add the pumpkin cubes and cook for a further 20-25 minutes or until the pumpkin is cooked and the sauce has thickened. Stir in the frozen peas during last 5 minutes of cooking. Serve with steamed jasmine rice.