1. Zest the lemon first, then cut it in half and juice it. Combine with the garlic, parsley, salt and pepper. Add the beef strips and marinate while you cook the vegetables.
2. Heat 2 tablespoons of vegetable oil in a pan over medium to low heat. Add the onions and cook slowly stirring occasionally for 20 minutes. If they start to brown, lower the heat. Stir in the sugar. Add the peppers and continue to cook for 10 minutes. Remove vegetables from pan and set aside.
3. Heat the remaining tablespoon of oil in the pan over medium to high heat. Sauté the beef strips for 8 minutes turning to brown all sides. Return the vegetables to the pan for 1-2 minutes to heat through.