Divide the ground beef into 4 portions and flatten each portion into a 6-inch wide circle. Divide the cheddar cheese between centres of each circle and fold up the edges around the cheese. Seal the meat around the cheese and lightly press into a 4-inch wide patty shape. Season with salt and pepper.
In a pan, heat a drizzle of oil over medium-high heat. Add the burgers and cook for 3-6 minutes per side, until cooked to desired doneness. (TIP: Inserting a thermometer into cooked meat should display an internal temperature of 160°F.) Don’t worry if some of the cheese leaks out, it’ll still taste delicious!
Charred Lettuce
Remove 2 outer leaves from each gem lettuce and set aside. Quarter the remaining lettuce lengthwise.
Heat a large pan over high heat. Add the lettuce quarters to the dry pan, cut-side down, and cook for 1-2 minute, until charred. Turn to char on other cut-side, then remove from the pan.
Tomato-Onion Jam
Core, seed and dice the tomato. Heat a drizzle of oil in the same pan over medium heat. Add the red onion to the pan and cook, tossing for 4-5 minutes, until softened. Add the diced tomato, sugar and half the balsamic vinegar to the pan.
Cook for another 4-5 minutes, until the tomatoes breaks down and the sauce is thick and jammy. Season with salt and pepper and transfer to a small bowl.
Balsamic Dressing
Preheat oven to 400°F. Halve the buns and toast them in the oven for 2-3 minutes, until golden brown. In a small bowl, whisk together half the mayo, remaining balsamic, a large drizzle of oil and as much minced garlic as you like.
Assembly
Layer the buns with burgers, tomato-onion jam and lettuce leaf. Spread with ketchup or mayo to taste. Serve with the charred romaine drizzled with balsamic dressing.