Preheat your oven to 450°F / 230°C. Pierce the eggplant all over with a fork, about 12-15 times, and place it in a roasting pan.
Bake for approximately 45 minutes, turning every 15 minutes, until the skin is charred and the inside is completely soft and pulpy.
Set aside to cool until you can handle it.
Once cool, peel the skin from the eggplant and discard it.
Place the smoky pulp in a bowl and mash it well with a fork.
Add the tamarind concentrate, salt, brown sugar, and red chili powder to the mashed eggplant.
Mix thoroughly until everything is well combined.
Taste and adjust the seasonings to achieve your perfect balance of hot, sweet, and sour.
Prepare the tempering (tadka).
Heat the oil in a small pan over medium heat.
When the oil is hot, add the asafetida. A few seconds later, add the mustard seeds. As soon as they begin to pop, add the dried red chili and the lightly crushed curry leaves—stand back as they will splutter!
Stir the tempering for a few seconds until the chili darkens slightly, then immediately pour the entire contents of the pan (oil and spices) over the eggplant mixture.
Stir well to combine. You can remove the whole red chili and curry leaves before serving if you prefer.
To serve, lightly toast the baguette slices.
Spread with a thin layer of butter, if using, then top with a generous spoonful of the eggplant gojju.
Serve immediately.
