Step 1Core tomatoes and cut in half lengthwise. Cut each half in 1/2-inch strips. Holding strips together, chop crosswise into 1/2-inch pieces. Place in large bowl and set aside.
Step 2Cut garlic into tiny strips lengthwise, then hold strips together and chop into tiny pieces crosswise. Add to tomatoes. Cut onion into 1/4-inch strips lengthwise. Holding strips together, cut strips crosswise into 1/4-inch pieces. Add to bowl. Peel cucumber and cut it lengthwise down center. Using teaspoon, hold cucumber half over sink and scrape out seeds. Cut each cucumber half in 4 strips lengthwise, then cut into 1/2-inch pieces crosswise. Add to bowl. Salt vegetables in bowl to taste and toss to mix vegetables. Taste and add more salt if salad tastes flat.
Step 3Place vinegar in small bowl, add oil and whisk to mix well, or use a spoon and stir briskly. Pour dressing over salad and toss to mix salad thoroughly.
Step 4Trim crust off bread with sharp knife and discard. Cut loaf into 1/2-inch cubes and add to salad. Toss salad to mix thoroughly. If it seems a little dry, add more olive oil and another spoonful or two of vinegar. Remove leaves from watercress stems and add to salad. Toss and serve.