In a bowl, whisk the eggs lightly with the fish sauce and sesame oil. Heat a wok with the peanut oil and when the oil is hot and starts to shimmer, quickly pour in the eggs. They will fluff up.
Use a wooden spoon to move the eggs around a little to cook evenly. Cook 3-5 minutes until golden brown underneath and almost set.
Turn off the heat and push the omelet to one side and pour off the oil. Add the sprouts and spring onions to the middle of the omelette and fold it over. Allow to sit for a minute to finish cooking inside, then slide the omelette onto a chopping board. Cut in half, place on two plates and serve, drizzled with a little oyster sauce.