In a deep, 2-quart, heat-resistant, non-metallic casserole, Place vegetable oil, onion and tomatoes and heat, uncovered, in Microwave Oven 4 minutes or until tender. Set aside.
In a small, heat-resistant, non-metallic bowl melt butter in Microwave Oven 30 seconds.
Blend in flour, salt, pepper and chicken stock.
Heat, uncovered, in Microwave Oven 3 minutes or untiI thick- ened and smooth. Stir occasionally.
Add sauce and cooked lamb to onion-tomato mixture. Stir to combine.
Heat, covered, in Microwave Oven 5 to 7 minutes or until almost heated through (time will depend on temperature of cooked lamb). Top with mashed potatoes and heat, uncovered, an additional 3 to 5 minutes or until serving temperature is reached.
Variation: Cooked beef chunks, ground beef or ground lamb may be substituted for cooked lamb.