Boil or steam the broccoli until soft. Add 2 tbsp. oil to large pot and sear the broccoli in a single layer until dark brown on one side. Remove with slotted spoon and repeat adding more oil until all broccoli is done. Season with salt and Set aside.
Reduce heat to medium add butter and remaining oil to pan. Add the onions and garlic and cook about 4 minutes. Add potatoes to pot and 4 cups of water. Season with salt.
Bring to a boil and cook until potatoes are tender. Add the chicken stock and broccoli. Cook until broccoli is tender. Add the lemon juice and zest. Puree with immersion blender until smooth
Taste and season if needed. Serve with grated Parmesan.