On a small baking sheet, toss the bread cubes with 2 tablespoons olive oil to coat well. Bake until golden brown and crispy on the outside but still with a bit of a chew in the center, about 12 minutes. Let cool.
Whisk together the vinegar, 2 tablespoons olive oil, and salt in a small bowl.
To a large bowl, add the celery, celery leaves, apple, fennel, Pickled Red Onion and arugula, and toss gently with two spoons. Add the toasted bread cubes and the dressing; toss again. Scatter the Gorgonzola over the top and serve.
Pickled Red Onion:
Mix together the red onion, vinegar and salt in a bowl. Set aside to pickle for 15 minutes, tossing occasionally.
Freeze the Gorgonzola for 4 hours, then use a vegetable peeler to shave thinly.