Simmer chicken stock in a medium saucepan over a medium high heat, or in a billy.
Add borlotti beans and let simmer 10-15 minutes, until tender. Add fennel, radicchio leaves, onion and garlic, let simmer 10 minutes, until tender. Season with salt and pepper to taste.
Use tongs to transfer beans, fennel and radicchio into four bowls. Pour oven stock. Break trout over the top of the soups in 3-4cm pieces. Scatter over shaved Parmesan or pecorino and freshly ground black pepper. Serve with slices of crusty bread, for dipping.