Melt the butter in a 4 quart saucepan over medium heat. Add the onion and garlic and sauté until soft and fragrant, 2 minutes. Add the spinach, toss to combine and season with salt and pepper.

Continue cooking the spinach until completely wilted, then add the chicken broth and avocado and bring to a simmer. Stir in the lemon juice and more salt and pepper if desired. Cover, reduce the heat to medium low and cook for 5 minutes, stirring occasionally.

Remove from the heat and stir in the half-and-half. Using a hand-held or conventional blender, purée the soup, taste and adjust the seasoning if needed.

To serve, ladle into bowls and garnish with the chopped scallions.