Using a mandoline or sharp knife, cut off the bulbs of your fennel, then cut down the center lengthwise, thinly slicing. Reserve about 2 to 3 tablespoons of fennel fronds, and roughly tear them apart.
Transfer your fennel and fronds to a large bowl, then toss in your thinly sliced celery and greens.
Season with kosher salt, freshly ground black pepper, extra-virgin olive oil and squeeze of lemon juice, toss to combine. Season to taste with more salt, pepper and lemon.