Fill a large bowl with ice and water. Bring a medium pot of water to a boil. Add half the spinach to the boiling water, stirring well until it wilts, about 15 seconds. Using a spider strainer or mesh spoon, transfer the spinach to the ice water and repeat the blanching process with the remaining the spinach.
Drain the spinach well and transfer it to a blender. Add 1 cup of water and blend into a fine, bright green purée.
In a large skillet set over medium-high heat, warm the oil until it shimmers. Add the cumin seeds and garlic and sizzle until the garlic browns, about 30 seconds. Add the onions and cook, stirring occasionally, until soft but not browned, about 8 minutes. (The garlic will make the onions look browner than they really are.)
Stir in the ginger, green chili, and turmeric and cook for 30 seconds. Add the spinach purée and cook, stirring constantly, for 5 minutes. Be careful; it will sputter like bubbling lava.
Reduce the heat to medium and add the cream, salt, and fenugreek leaf powder. Return to a boil and cook for 5 minutes. Serve hot.