*Garlic Cayenne paste: 4 cloves of garlic pulverized, 1 teaspoon cayenne pepper (more if you want), 1 teaspoon ground cumin mashed into the garlic. It helps to put a little sea or kosher salt in to act as an abrasive.
Heat oil over medium in large saucepan, add mustard seeds. Once seeds begin to pop, add potato and eggplant and stir-fry for a few minutes (if the start sticking a lot, add a little more oil).
Add the dry spices and salt and continue stir-frying for a few minutes until all spices are well blended; mix in garlic paste, breaking up any lumps.
Add 3/4 cup water (you may need more, maybe not; 3/4 cup is a good starting point), lower heat and cover pan.
Cook about 15 minutes or until the eggplant and potatoes are soft. If the sauce begins to dry up add a little more water.