Bring a large pot of salted water to a boil. Add the orzo and cook as the label directs, adding the celery slices during the last 2 minutes. Reserve 1/2 cup cooking water, then drain. Set aside. Meanwhile, toast the pine nuts in a dry skillet over medium heat, stirring occasionally, until golden brown, 3 to 5 minutes.
Heat the olive oil in a large skillet over medium-high heat. Add the garlic and cook until lightly golden, about 3 minutes. Add the olives and red pepper flakes; stir to combine. Stir in the orzo and celery slices along with 1/4 cup of the reserved pasta cooking water. Cook, stirring, until the water is mostly absorbed and the pasta is coated, adding more pasta cooking water if needed, about 2 minutes. Stir in the celery leaves, herbs and sardines; season with salt and pepper. Drizzle each serving with olive oil and top with the toasted pine nuts and cheese.