Moroccan Beef Tajine with Potatoes

Ingredients:
1 pound (500 grams) beef (or lamb) meat cut into medium sized pieces
1 medium sized onion (grated)
1 teaspoon salt
1/2 teaspoon ground ginger
1/4 teaspoon black pepper
1/4 teaspoon ground turmeric
A small pinch of saffron threads (optional)
2 tablespoons olive oil
3 garlic cloves (crushed)
4 large white potatoes
10-12 green cured olives (Mediterranean or Greek style)
3-4 cups of water (around 1 liter)