Heat the oil in a broad non-stick pan, add the garlic, ginger and green chillies and sauté on a medium flame for 30 seconds. Add the spring onion whites and greens and sauté on a medium flame for 1 minute. Add the capsicum and sauté on a medium flame for 1 minute. Add the mushrooms and salt, mix gently and cook on a medium flame for 2 to 3 minutes ,while tossing occasionally. Add the cornflour mixture and pepper, mix gently and cook on a medium flame for 2 minutes, while stirring occasionally. Serve immediately.