Place frozen spinach in a blender or food processor, drizzling it with the boiling water as you go so it wilts and you can fit it all in. Process until very finely chopped, but not pureed. Set aside.
Heat ghee or butter in a large pot over medium heat. Add chopped onion and cook, stirring often, until very soft and golden - 20-25 minutes. Reduce the heat if onions are browning - low and slow is the goal.
Add the ginger and garlic and cook until fragrant, about 1 minute. Add the turmeric, cumin and fenugreek powder and cook 1 minute more. Stir in the spinach, bring to a simmer, then add cream or yogurt, lemon juice and salt to taste. Gently stir in paneer and heat through.