Cut tomatoes in medium size pieces. Take 1 tsp oil in a pan, add urad dal and chana dal and on medium flame cook until they get light brown color. Now add curry leaves, asafoetida, green chilies, red chilies, ginger, tomatoes and cook until tomatoes become soft. Now add spinach and cook for 1-2 minutes so that the rawness disappears.
When all the spices cools down little, place all ingredients in the mixture grinder and add green coriander leaves, black salt, normal salt and ¼ cup water. Grind all ingredients finely. Add lemon juice in the chutney.
Take out chutney in bowl and add tadka in it. Preheat a pan; add 1 tsp oil and rei. Add this in chutney and mix well. Palak chutney is ready.