For the potatoes, boil the potatoes in a pan of salted water for 8-10 minutes, or until just tender.
Add the sweetcorn kernels and continue to boil for a further 1-2 minutes, then drain well and set aside.
Heat the oil in a large frying pan over a medium heat. Add the onion and fry for 4-5 minutes, or until softened.
Add the chorizo and continue to fry for 3-4 minutes, or until crisp and coloured.
Increase the heat and add the cooked potatoes and sweetcorn. Stir just enough to coat the potatoes in the oil released by the chorizo. Remove the pan from the heat and set aside. Keep warm.
For the tartare sauce, in a bowl, mix together the egg yolks, salt, mustard and white wine vinegar until smooth and well combined.
In a separate bowl, mix together the sunflower oil and olive oil, then add the oil mixture to the egg mixture, a drop at a time, whisking continuously, until the mixture has thickened and emulsified.
When all of the oil mixture has been incorporated into the egg mixture, season, to taste, with salt and freshly ground black pepper. Chill in the fridge until needed.
When the tartare sauce is ready to serve, stir the remaining tartare sauce ingredients into the emulsified egg and oil mixture until well combined.
For the steaks, heat the oil in a large heavy-based frying pan over a high heat. When the oil is smoking, add the sirloin steaks (add them in batches if they do not all fit comfortably in the pan).
Fry the steaks for 2-3 minutes on each side (for medium-rare), or for longer if you prefer your meat well-done. Remove from the pan and set aside to rest for 4-5 minutes.
To serve, place one of the steaks into the centre of each of four serving plates. Spoon the potato and chorizo mixture alongside. Place a spoonful of the tartare sauce on top of the potato and chorizo mixture.