Combine the garlic, chilli, ginger, soy sauce, mirin, and fish sauce and oil in a bowl. Keep some of the marinate aside in a small container to use a dressing.
Add the striploin steak to the main marinade bowl and marinate for five - 30 minutes, covered at room temperature or in the fridge if a longer marinate is desired.
Preheat a large sauté pan, over medium high heat and add the steaks. Cook for two to five minutes on both sides until ready. Add the new onions and grill for a further two to three minutes.
In the meantime, prepare the salad, place the salad leaves, carrots, green peppers and add the dressing and season with some salt and pepper. Toss to combine and divide into serving plate.
Remove the steaks from the heat and carve. Place the sliced beef over the salad and the grilled onion. Serve immediately.