Step 1Tear the ciabatta into rough pieces and put it in a bowl with water to cover. Soak for at least 15 minutes.
Step 2Cut cherry tomatoes in halves or quarters, depending on their size. Cut heirloom or other large tomatoes into three-quarter-inch slices, then cut the slices into quarters. Place them in a bowl and season with the salt, pepper, red wine vinegar and olive oil. Set aside for at least 15 minutes to allow the salt to pull some of the juice from the tomatoes.
Step 3Pull the bread from the bowl a handful at a time and squeeze out as much water as you can. Crumble the bread into a serving bowl. Stir the tomato mixture into the bread, working the mixture to moisten as much of the bread as possible with the liquid from the tomatoes. Stir in the red onion. (The recipe can be prepared to this point up to 8 hours ahead and refrigerated, tightly covered.)
Step 4When ready to serve, remove the bread salad from the refrigerator and taste it. You may need to add more salt or more vinegar. Add the pine nuts and arugula to the bread salad and toss lightly to mix thoroughly. Serve immediately.