Cut the zucchini and squash in half lengthwise, then slice crosswise into ½-inch (1. 25 cm) thick pieces.
In a small bowl, whisk together the yogurt, turmeric, coriander, salt, black pepper, and chili powder until smooth.
Set aside.
Place a medium frying pan over medium-high heat and add the oil.
Once the oil is hot, add the asafetida, mustard seeds, and cumin seeds.
Let them sizzle for a few seconds until the mustard seeds begin to pop.
Immediately add the chopped zucchini and squash to the pan.
Stir and fry for 3-4 minutes, until the vegetables are tender-crisp and just beginning to brown at the edges.
Reduce the heat to medium-low.
Add the spiced yogurt mixture one tablespoon at a time, stirring for about a minute after each addition to prevent curdling and allow the sauce to thicken.
After all the yogurt is incorporated, add 1 tablespoon (15 ml / 0.5 fl oz) of water, stir a final time to create a light sauce, and taste for salt.
Turn off the heat and serve immediately.
