In a medium bowl, combine the finely diced shallot and celery with lemon zest, juice and salt. Stir to coat the vegetables in the lemon juice and let sit for 5-10 minutes at room temperature.
To finish the dressing, slowly whisk in the extra virgin olive oil until the dressing is emulsified.
To assemble toss the celery leaves, if using, frisée, arugula and white beans together in a large bowl. Add the dressing and toss to coat. Add the IASA branzino and gently toss to combine, making sure not to break up the fish too much. Taste and adjust seasonings, as necessary.
To serve, transfer the salad to a serving platter, drizzle with more extra virgin olive oil and finish with a sprinkle of fleur de sel.