Heat oil in large pot not a pan, add cubed meat & saute until golden brown 4-5 mins on med heat.
Add onions, chillies, garlic, curry powder, turmeric, spices & mustard seeds. Stir in 2-3 mins till an aroma develops.
In a bowl mix together tomato paste, sugar, lemon juice & stock. Then pour half the mixture over the meat gently & cook on med-high stirring constantly until almost dry. Reduce heat & pour over balance of mixture & mix thoroughly. If too thick add a little water.
Cover with lid & let simmer for 1/2 hour. Stir occasionally.