Wash the tomatoes and dry them thoroughly. Lay the washed tomatoes on a kitchen towel and let dry for about 1 hour. Then wipe them with a clean kitchen towel. Chop each tomato into big chunks and place them in a large non-reactive bowl - preferably glass or ceramic.
Add salt and mix well with a wooden spoon. Cover and set aside for 48 hours. Take a peek after 24 hours and mix to evenly combine the ingredients.
Place a sieve or colander in a large bowl and keep ready.
After 48 hours, drain the tomatoes into the sieve and reserve all the soaking liquid. Set aside for at least 1 hour for all the liquid to drain.
Place the drained tomato slices onto a parchment or silicone mat lined baking sheet.
Pour the soaking liquid into a glass or ceramic bowl and add tamarind to it and let it soak for at least 30 mins.
Preheat oven to 250°F.
Bake the tomatoes for 2-2½ hours or until the slices appear to be dry. They do not have to be very dry. The main is to take as much moisture out of the tomato as possible and this simulates traditional sun-drying process.
After about an hour, place the soaking liquid in the oven along with the tomatoes and bake for 1 hour. Remove baking sheet and the bowl from the oven and set aside to cool completely.
Once the tomatoes and the liquid are completely cooled, cover and set them aside overnight.
Next day, grind the tomatoes with the tamarind liquid into a smooth paste. Pour this into a glass bowl and add red chili powder, ground fenugreek and mix well. This is the tomato concentrate. Transfer it into clean, sterilized glass jars and refrigerate until ready to use.