In a soup pot over medium-low heat, cook pancetta, stirring, until golden and crisp, 4-6 min. Add onion and garlic and cook, stirring, until translucent, 4-6 min.
Reduce heat to low, add sage, ceci beans, broth and rosemary and cook, stirring, 20 min. Remove rosemary and, using a stick blender, puree soup until smooth and blended. (If using a tin-lined copper pot, be careful not to touch the bottom or sides of the pot with the blender.) Season with salt and pepper.
Ladle soup into individual bowls and top each serving with a spoonful of pesto.
