To make the aioli, break the egg into the food processor and add the garlic, half a teaspoon of salt and the vinegar, sugar, mustard and half of the olive oil. Secure the lid and blend for 10 seconds.
Leave to stand for a couple of seconds, then turn on again and pour the remaining olive oil and sunflower oil through the feeder tube in a thin steady stream. This should take 25-30 seconds.
Switch off the machine, take off the lid, scrape down the sides, and whizz again for 2-3 seconds, then season to taste. Transfer to a bowl, then cover and chill until needed.
Heat a cast iron griddle pan until searing hot and lightly smoking. Rub the sirloin steak all over with the cut garlic clove and then rub in a tablespoon of the olive oil. Add to the griddle pan and cook for about 8 minutes on each side until nicely marked but still rare on the inside.
Season with salt, then loosely cover with tin foil and set aside in a warm place.
Wipe the griddle pan clean. Place the rest of the olive oil with some seasoning in a shallow dish and add the spring onions, tossing to coat.
Place on the griddle pan and cook for 6-8 minutes until nicely marked turning regularly.
Meanwhile, smear the cut little Gem lettuces in the rest of the olive oil and add to the griddle pan. Cook for 2 minutes on each side until well marked but still holding their shape.
Arrange the little Gem lettuce halves on a platter and add the spring onions and sun blushed tomatoes. Drizzle over the balsamic vinegar.
Cut the steak into ½ inch slices and arrange on top. Whisk any juices from the sirloin into the aioli and drizzle a little on top of the salad. The rest of the aioli can be served in a separate small dish on the table.
Serve with crusty bread.