Heat the oil in a large, deep skillet over medium heat until shimmering. Add the onion and cook, stirring frequently until soft and golden, about 15 minutes (reduce the heat to medium low, if necessary, to prevent the onion from burning). Stir in the ginger and garlic and cook for 2 minutes. Add the eggplant, yellow squash, zucchini, and 1/2 tsp. salt; stir to coat thoroughly. Cook over medium heat , stirring occasionally, until the vegetables are barely tender, 7 to 10 minutes.
Stir in the garam masala, coriander, cumin, turmeric, chile, 1 tsp. salt, and a few grinds of pepper. Cook until the spices are fragrant, 1 to 2 minutes. Pour in the chickpeas and their liquid, the tomato sauce, coconut milk, and 2 Tbs. of the cilantro.
Raise the heat to medium high and bring the stew to a boil. Reduce the heat to medium low and simmer, uncovered, until the eggplant and zucchini are completely tender but still hold their shape, and the sauce has thickened, 15 to 20 minutes.
To serve, ladle the stew into shallow rimmed bowls and sprinkle with the remaining 1/4 cup cilantro and the toasted coconut (if using).