I made the same chutni with ordinary onions not Madras onions or vengayam as I do not buy vengayam regularly.
Ingredients : 4 medium onions cleaned & roughly chopped,6-8 byadgi red chillies broken up into pieces,, 2 tbsp oil,1/8 tsp methi seeds1/2 tsp urad dal,1 lemon sized ball of tamarind,1 tsp mustard seeds,a few curry leaves,a pinch of hing ,salt to taste
Method : Take 1 tbsp oil in a kadhai & put on the gas.Put the byadgi chillies & as soon as they turn dark red,remove the chillies from kadhai & keep aside.Add chopped onions. Saute the onions till they become pink.Put off gas.Add the tamarind & mix it with the sauteed onions in the kadhai for 1 minute.Take the sauteed onions,tamarind mix & byadgi mirchis into the mixer & grind them into paste.Add salt & mix nicely.Keep aside.
For seasoning ,take 1 tbsp oil & heat it.Add mustard seeds.After they pop up,add urad dal.As soon as they turn golden ,add methi seeds..After they pop up,add curry leaves.Put off gas.Add hing .After this ,add this seasoning to the onion chutni.
This chutni is especially useful when one runs out of fresh coconut .Moreover it is quite tasty & especially so for idlis & dosas.