Add 1 tbsp of the oil to a large frying pan over a medium heat. Add the onion and cook for 6-7 minutes, until softened and just colouring. Add the garlic and cook for a further minute until fragrant. Scrape into a large bowl.
Add the mince and plenty of seasoning, and combine. Separate into 4 equal portions, and use your hands to gently shape into 4 burgers.
Heat a griddle pan over a medium heat until hot. Brush each burger with the remaining olive oil and then add to the pan (you might need to do this in two batches, depending on the size of your grill). Cook without disturbing for 4 minutes, until the underside is nice and charred. Turn over and cook for a further 3 minutes. Transfer to a plate.
Wipe the pan clean and then add the burger buns, underside down. Cook for 1 minute on a medium heat until just toasted. Remove and layer up your bottom buns with the lettuce, tomato slices and burgers. To serve, sprinkle over the crumbled feta, then top with the other half of the bun.
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