Lyn's beef stir-fry

Ingredients

• 3 tbsp oyster sauce
• 3 tbsp dry sherry
• 3 tbsp soy sauce
• 2 garlic cloves , minced, or 2 tsp bottled chopped garlic
• 2 tsp grated fresh ginger , or bottled chopped ginger
• 1/2 tsp freshly ground black pepper
• 500 g beef stir-fry strips , or inside round, cut into thin bite-size strips
• 1 yellow or red bell pepper
• 1 small red onion
• 2 green onions
• 1 tbsp cornstarch
• 2 tsp dark sesame oil
• 2 tsp vegetable oil