In the same wide kadhai used to cook the eggplant and jackfruit, add the oil for tempering. When it’s very hot, add the mustard seeds and hing—once the seeds splutter, add the cooked-mashed eggplant and jackfruit.
Mix well and fry for a minute. Then add the tamarind water, and jaggery and salt to taste. Wash out the tamarind with a little more water if need be and add that, too.
Bring the gotsu to a slow boil, and continue to cook and stir for a few minutes.
Now add the spice powder, mix well, and continue to stir. Also add the 1 teaspoon turmeric and 1 teaspoon red chilli powder [if using] now. The gotsu will thicken quickly at this stage, so keep your eye on it and keep scraping down the sides of the pan.
You can mash the eggplant and jackfruit further while cooking, or you can simply mix to keep a slightly rough texture—it’s up to you.
Once the gotsu seems to become homogenous (you can’t tell the difference between the eggplant and the jackfruit any more), add the ½ cup sesame oil. Continue stirring and cooking on medium-low heat.
The gotsu is done when the oil starts to emerge from the sides of the pan so it looks like the liquid is frying rather than boiling.
The fragrance of the coriander seeds should be prominent, the gotsu itself should be a mix of all 6 tastes, but predominantly sour-spice-sweet-salt (roughly in that order), and should look visibly reddish. Even testing by smell is tantamount to tasting/consuming—for a naivedyam that’s not done, but really when you have this sense of the gojju you know you’re done.
The gotsu will keep well, refrigerated, for a few weeks.