Moroccan Tomato Cucumber Salad

Ingredients:
0.6 pound (300 grams) fresh tomatoes
1 cucumber
1 small sweet yellow onion
2 tablespoon finely chopped cilantro
Juice of 1 lemon (to taste)
Peel of 1 quarter preserved lemon (optional)
1 tablespoon olive oil
Spices: 1/2 teaspoon slat
1/4 teaspoon black pepper
1/4 teaspoon ground cumin (to taste)