Heat 3 tablespoons olive oil in a large saute pan. Add the bread cubes and sprinkle with salt; cook over low to medium heat, tossing frequently, for 5 to 10 minutes, until nicely browned. Add more olive oil as needed.
Place the cucumber, red pepper, yellow pepper, tomatoes and red onion in a large bowl.
For the vinaigrette, whisk together the garlic, oregano, mustard, vinegar, 1 teaspoon salt and the pepper in a small bowl. While still whisking, add the olive oil and make an emulsion. Pour the vinaigrette over the vegetables. Add the feta, olives and bread cubes and mix together lightly. Set aside for 30 minutes for the flavors to blend. Serve at room temperature.