In a medium saucepan, bring the stock to a simmer over medium heat.
Meanwhile, in a large pot, melt the butter over medium heat. When it foams, add the onion, shallots and chopped garlic. Partially cover and cook, stirring occasionally, until the vegetables soften, about 5 minutes. Add the flour, whisk for a minute or so, then whisk in the warm stock. Add the spinach and stir until wilted. Simmer 10 to 15 minutes. Add the lemon juice and season with salt and pepper.
Using an immersion blender or working in batches in a high-powered blender or food processor, puree the soup. If using a blender or food processor, return the soup to the pot. Stir in the heavy cream and simmer over low heat 5 minutes. Season to taste with nutmeg and a bit of cayenne or ground red pepper.