Preheat the oven to 400 degrees F. Soak the onion in a bowl of cold water while you prepare the salad.
Working in batches, cook the pancetta in a single layer in a large skillet over medium heat until crisp, about 4 minutes per side. Transfer to paper towels to drain; reserve the drippings.
Combine 1 tablespoon each olive oil and pancetta drippings in a small bowl. Toss with the bread cubes on a baking sheet and sprinkle with 1/4 teaspoon each salt and pepper; spread in an even layer. Bake, stirring once, until lightly toasted, about 15 minutes.
Drain the onion and chop all but 6 slices pancetta. Add to a large bowl with the tomatoes, arugula, basil, olives, bread, vinegar and the remaining 3 tablespoons olive oil. Toss and season with salt and pepper.
Divide the salad among bowls. Top each with some ricotta salata and a piece of reserved pancetta.