Combine the onions, garlic, green peppers and tomatoes. Force through a sieve or puree in a blender. Add the salt, pepper and paprika. Add the olive oil gradually, beating steadily. Add the vinegar and water and stir well. Season to taste. Refrigerate and chill for at least two hours, using a wooden or glass bowl; do not use a metal bowl. Add the cucumber slices before serving. If desired, slices of toast rubbed with garlic may be served with the Gazpacho.