Wash the eggplant and pat it completely dry. Make 3 to 4 deep longitudinal slits into the eggplant. Insert a peeled clove of garlic into each slit; this infuses the eggplant with a roasted garlic flavor from the inside out.
Brush the eggplant lightly with oil. Place it directly over a medium flame on your gas stove. Roast for 10-12 minutes, turning occasionally with tongs, until the skin is completely charred and papery, and the eggplant collapses inward.
Transfer the roasted eggplant to a plate and cover it with a bowl for 5 minutes to steam. Once cool enough to handle, gently peel off the charred skin. Mash the tender pulp thoroughly with a fork, preserving any roasted garlic cloves.
Heat mustard oil in a pan until it reaches smoking point, then lower the heat slightly. Add cumin seeds and let them sizzle. Add the chopped onions and cook for 5-6 minutes until soft and golden brown.
Stir in the minced garlic, ginger, and green chilies. Sauté for 2 minutes until fragrant. Add the chopped tomatoes, turmeric, Kashmiri red chili powder, coriander powder, and salt. Cook until tomatoes turn completely soft and oil begins to separate at the edges.
Add the mashed eggplant pulp to the pan. Mix everything thoroughly. Cook on medium-low heat for 7-8 minutes, stirring frequently, to allow the smoky eggplant to absorb the spices. Finish by stirring in garam masala and garnishing with fresh coriander.