Preheat the oven to 350 degrees F. Heat the oil in a large skillet set over medium-high. Season the lamb with salt and pepper. Brown the lamb, in batches, and then set in a large casserole dish. Add the onion to the skillet and cook 2 to 3 minutes. Mix in the curry powder, flour and paprika and cook 1 minute more. Slowly, while stirring, mix in the coconut milk. Add the diced tomatoes and brown sugar, bring to a simmer, and then pour over the lamb. Cover and cook in the oven for 45 minutes. Mix in the potatoes, cover and cook 30 minutes more, or until the lamb and potatoes are tender. Stir in the peas and cilantro or mint and heat them through a few minutes, and then serve.