In a large stockpot, heat the oil over medium heat. Add the onions and saute until soft and translucent, about 3 minutes. Add the artichoke hearts, season with salt and pepper and saute another 5 minutes. Add the broth, bring to a simmer and cook about 20 minutes, stirring occasionally. Puree until smooth with a hand blender or in a blender in small batches. Whisk in 3/4 cup yogurt and season with salt and pepper.
In a small bowl, stir together the pesto sauce and lemon juice. Ladle the soup into bowls, top each with about 1 tablespoon yogurt and drizzle with the thinned pesto sauce.