For the beef, place the soy sauce, chili, garlic oil and oil in a blender or food processor and blend until smooth. Place the beef in a non-reactive bowl and pour the marinade over the beef.
Cover with cling film and leave the beef to marinate for several hours or overnight in the fridge. Turn the beef in the marinade from time to time. When you are ready to cook, remove the beef from the fridge and bring it to room temperature. Preheat the oven to 220C/430F/Gas 7.
Heat a large ovenproof frying pan until hot, carefully lift the beef out of the marinade (discarding the marinade), place the beef in the pan and sear on each side until brown. Place in the oven and roast for 20-25 minutes.
Remove the beef from the oven and allow it to cool in the frying pan. For the salad, place the soy sauce, ginger, chili, lemon juice, fish sauce and sugar in a bowl and whisk to combine.
To serve, arrange the spinach leaves and shredded vegetables on a plate. Thinly slice the beef fillet and arrange it over the salad leaves. Drizzle with the dressing and serve.