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Stir the spinach and garlic occasionally to help them cook more evenly and prevent them from burning. Cook until the spinach is wilted, about 2 to 3 minutes. Once the spinach has wilted, transfer the mixture to a blender or food processor. Add some hot water and puree until it is smooth. If the spinach is not mixing smoothly enough, use a rubber spatula to scrape any unblended spinach down the sides and towards the bottom. Set the mixture aside when it is completely smooth. Make sure that the saucepan is large enough to fit the rest of your ingredients, including the milk and pureed spinach. Occasionally stir the mixture with a spatula. You will end up with a roux-like paste, which you will eventually thin out. This will help create a thicker soup. Use a whisk to help the roux dissolve and break up any clumps or lumps. Open up the blender or food processor, and pour the pureed spinach into the saucepan. Whisk everything together to combine, then cook until the soup thickens, stirring occasionally. If you need to, add in some more salt, cayenne pepper, and/or black pepper. For extra flavor, garnish it with Gruyere cheese. You can also use other types of cheese as well.

Summary:
Heat the oil in a large skillet over medium heat, then add the spinach and garlic. Puree the mixture in a blender or food processor with ¼ cup (60 milliliters) of hot water. Cook the onions in butter over medium heat in a saucepan until the onions start to soften. Sift the flour into the mixture, and cook for another 2 minutes. Pour in the milk, salt, and pepper, and cook over medium heat for 5 minutes, stirring constantly. Add the pureed spinach and cook until it thickens, about 3 to 5 minutes. Taste the soup and make any necessary adjustments. Pour the soup into bowls and serve.