1. Heat oil in a large frying pan.
2. Add onions and sauté over medium-low flame for one hour, stirring occasionally. Add the garlic and sauté two to three more minutes, turning up the heat for the last minute.
3. Fifteen minutes before the end, preheat oven to 350 degrees Fahrenheit (175 degrees Celsius).
4. Remove the onions from the heat and add the spinach and seasoning. Mix together well. Pour into the pie crust.