Clean the spinach leaves and wash them to remove any dirt or debris. If spinach leaves are big, chop them roughly.
Dissolve 1/2 tablespoon corn starch in 1/2 cup milk. Make sure that there are no lumps of flour.
Heat 1/2 teaspoon oil and 1/2 teaspoon butter together in a deep saucepan or a kadai over medium flame. Add chopped ginger and garlic; sauté for few seconds. Add chopped onion.
Sauté until onion turns light pink.
Cook until spinach wilts.
Add a cup of water, sugar and salt. Stir well and bring the mixture to boil. Let it boil for 3 minutes. Sugar is an optional ingredient in this recipe. It is added to retain rich green color of spinach and prevent it from turning dark.
Turn off the flame and let the mixture cool for 5-10 minutes. Make smooth puree of the cooked mixture by either using a stick blender (hand blender) or a mixer grinder (or blender). Transfer the mixture to the same pan after making the puree.
Add dissolved cornstarch and stir continuously for 1 to 2 minutes over medium flame.
Add black pepper powder.
Let the mixture cook for 4-5 minutes. Taste for the seasonings and adjust it per your taste.
Turn off the flame and transfer spinach soup to serving bowls. Serve it hot with bread croutons or garlic bread sticks.