How to Make It
Combine lettuce, celery, and apple in a large bowl. Reserve 3 tablespoons each of pancetta and blue cheese. Add remaining pancetta and blue cheese to lettuce mixture.
Combine fresh lemon juice, red onion, mustard, reserved pancetta drippings, 1⁄2 teaspoon of the pepper, and 1⁄4 teaspoon of the salt in a small jar with a lid. Place lid on jar, and shake well to combine, about 30 seconds.
Add olive oil to jar. Place lid on jar, and shake until emulsified. Pour dressing over salad, and toss to combine. Sprinkle salad with reserved pancetta and blue cheese and remaining 1⁄4 teaspoon each salt and pepper. Serve immediately.