Heat the oil in a large pot. Add the garlic and stir for 30 seconds, then add the potatoes and continue to cook, over a medium-low flame, about 4-5 minutes until they begin to color just slightly. Keep stirring to ensure the garlic does not burn.
Add the eggplant and stir for another 2 minutes. Now add the bell peppers and salt and stir to combine.
Add the tomato puree, garam masala and turmeric, if using. Stir well and allow the puree to cook for about 5 minutes or until it starts to express the oil.
Add the fava beans with any remaining cooking water and stir well to combine. Add water if the stew is too thick. Bring to a boil, cover, lower heat so the stew just simmers, and let it cook 30 minutes or until all the vegetables are really tender.
Check salt and turn off the heat. Add the spring onions and coriander and mix in.