Heat butter and oil in a large Dutch oven or stock pot over medium-high. Add broccoli stalks, potatoes, onions, and garlic; season with salt and pepper. Sauté vegetables until beginning to soften, about 8 minutes.
Stir in broth, Dijon, and zest, bring to a boil. Then add in the Lucky Iron Fish/Leaf and continue boiling for 10 minutes.
After 10 minutes, the soup is iron enriched and the vegetables should be tender. Turn off heat. Remove the Fish/Leaf and wash it.
Purée soup with an immersion blender, or purée in batches in a standard blender. Stir in lemon juice, lemon zest, and season with salt and pepper.
**If you want to make croutons from scratch, just toss roughly torn pieces of bread in olive oil and spices of your choice. Bake at 400˙F for 7 minutes until crispy.