Prepare a rimmed baking sheet by lining with parchment paper. Adjust oven rack to middle position and preheat oven to 400 degrees. In a very large bowl, toss bread cubes with 2 tablespoons olive oil and 1/4 teaspoon salt.
Arrange bread cubes in a single layer on the prepared baking sheet and toast in oven until cubes turn light golden brown, about 15 minutes, stirring halfway through. Cool completely.
Meanwhile, in a large bowl, toss tomatoes and 1/2 teaspoon salt together. Transfer to colander and set over the large bowl. Allow to drain about 15 minutes, stirring occasionally. Reserve tomato juice.
Set drained tomatoes aside. Into the reserved tomato juice, whisk in vinegar, 1/4 teaspoon pepper, and the remaining 6 tablespoons olive oil. Add bread cubes, toss to coat and allow to stand for 10 minutes, tossing occasionally.
To the large bowl, stir in cucumbers, shallot, basil, and drained tomatoes. Season to taste with salt and pepper and serve immediately.
Baguette or Italian bread loaf: Panzanella is a brilliant vehicle for day-old bread (score more ideas to put this to great use below!). If you're starting with stale bread cubes, omit step 1 and jump right into step 2. Otherwise a fresh, crusty loaf will do.
Tomatoes: Root-to-stem eating at its best, I call for reserving the juice that drips off the cored, diced tomatoes to be strained and mixed into the Panzanella salad dressing. The firm tomato pieces will hold up beautifully in the salad with a little less moisture.
Cucumber: if using a hothouse cucumber, do not peel. Either way, remove the seeds, as they tend to make the salad too wet.
Basil: I call for fresh basil only, but if your herb garden is overflowing with parsley, oregano, or thyme, feel free to toss those in as well.
Yield: This recipe makes 8 2-cup servings, ideal as a starter or side dish.
Make ahead: The bread will become soggy if stored overnight, so Panzanella is best served immediately and enjoyed as close as possible to the time you assemble it.