If you are using fresh spinach, wash it in several changes of water to remove the grit. Coarsely chop the leaves,  discarding the stems, and set aside.

In a large skillet,  heat the oil over medium heat. Add the cumin seeds and cook until browned, about 15 seconds, Add the garlic, ginger, and onion and cook until the onions are translucent, about 3 minutes. Add the cremini mushrooms and cook until they release their liquid and are lightly browned, about 6 minutes.

Add the spinach ½ cup at a time, waiting to add the next addition until the greens have completely wilted,  When all the spinach has been added, stir in the coriander and salt.  Add the boiling water, reduce heat to low, cover partially, and cook until the greens are tender and soft,  about 10 minutes.

Uncover and cook until most of the liquid has evaporated and the oil begins to rise to the top of the greens and glazes them. Stir in the yogurt and continue cooking until creamy, about 5 minutes. Stir in the garam masala and season to taste with salt.