1. In a mixing bowl, whisk the eggs until well beaten.
2. Add the bean sprouts, chopped onions, bell peppers, and cooked shrimp or chicken (if using) to the eggs. Mix well.
3. Stir in the soy sauce.
4. Heat the vegetable oil in a frying pan over medium heat.
5. Pour a portion of the egg mixture into the pan, forming a small omelette. Cook until the edges are set and the bottom is golden brown, about 3-4 minutes.
6. Flip the omelette and cook the other side until golden brown, about 2-3 minutes more. Repeat with the remaining egg mixture.
7. Serve hot with additional soy sauce if desired.