1. Preheat the oven to 200°C. Heat 1 tbsp oil in a frying pan over high heat. Season the beef with salt and pepper and sear until completely browned, about 5 minutes. Add the lemon halves, cut side down, for the last minute. Remove the fillet and lemon halves and transfer the beef to an oven tray. 

2. Roast the beef in the oven until cooked to your liking, about 20 minutes for rare. Remove from the oven and set aside to rest for 10 minutes. 

3. Turn the largest element on your stove top to medium/high heat. Using tongs, flame-grill the chillies, garlic and white part of the spring onions, turning constantly until blackened all over. (If you have an electric or induction stove that doesn't produce a flame, you can blacken the vegetables in a heavy grill pan over high heat.) Place into a bowl and cover with plastic wrap for 10 minutes. 

4. Remove the skin from the chillies and discard stems and seeds. Peel the garlic and remove outer layer of the spring onions. Finely chop the chillies, garlic and spring onions together, then add to a bowl with the remaining 2 tbsp oil, chopped green spring onion, parsley, caster sugar and juice from the lemon. Stir until the sugar dissolves. Season with salt and pepper. Slice the beef and serve with the salsa. 

5. Serve with romesco salad (optional)

Note: If you want to get a good sear on meat, fish or vegetables in a frying pan, the process is really very simple. Get the pan very hot, don't use too much oil, and don't muck around with it too much. The more you prod and poke and flip your food in a pan, the less it will sear.