Finely chop the onion, celery, and garlic. Melt butter in skillet and cook the vegetables until softened but not browned. Stir in curry powder and thyme; cook, stirring for 1 minute.
Stir in chicken stock and tomatoes; simmer for two minutes. Season with salt and pepper; add more curry powder if desired. For smooth sauce, puree in food processor and pour back into the skillet.
Add the lamb* to the skillet; cover and simmer until the lamb is headted through, about 10 minutes, adding a little more stock if the sauce is too thick. Serve over rice.
*if you are not using leftover lamb it will need to be browned and stewed in stock for 1-1/2 hours until tender.