Wash the swiss chard, then separate the stems from the leaves. Finely chop the stems and roughly chop the leaves, keeping them in separate piles.
Heat the olive oil in a large pot over medium heat.
Add the onion, sliced fennel and chopped swiss chard stems. Cook, stirring occasionally, for 8 to 10 minutes, until the vegetables are soft but not browned.
Stir in the garlic, sea salt, and black pepper. Cook for 30 to 60 seconds, until fragrant.
Pour in the bone broth and water. Bring to a simmer and cook for 15 minutes, so the vegetables release their flavor into the broth. Stir in the swiss chard leaves. If using seafood, add the calamari and shrimp now. Cook gently, without letting the soup boil, for about 5 minutes, until the seafood is cooked through and the leaves are wilted.
Serve hot with a squeeze of fresh lemon juice, topped with green onion.
