In a large pot, melt 4 tablespoons butter over medium heat. Add 1 large chopped onion and cook until softened, about 5 minutes. Stir in 1 tablespoon flour and cook, stirring, for 1 minute. Add one 9-ounce bag spinach and cook, stirring, until wilted; repeat with a second bag of spinach, then remove from the heat. Add 2 1/2 cups vegetable broth and, using an immersion blender or food processor, puree until smooth. Return to medium heat and cook until just beginning to boil, about 2 minutes. Meanwhile, in a large bowl, whisk 1/2 cup heavy cream with 2 egg yolks. Slowly whisk a ladleful of hot soup into the cream mixture and repeat with another ladleful; stir the cream mixture into the pot, season with salt and pepper and heat through. Just before serving, stir in the grated peel of 1 lemon.