Heat one tablespoon of olive oil in a frying pan and gently cook the onions until translucent for 5-7 minutes. Put aside to cool.
Place the beef mince in a large bowl and add the Dijon mustard, sun-dried tomato paste, thyme and pepper.
Drain any liquid from the onions, and mix well with the beef using as stiff spoon.
Form your burgers into flat patties, using approximately 60g for children and 120g for adults. Remember, they will thicken up in cooking, so make them flatter than you want them to be when cooked.
Sprinkle salt on either side of each patty that needs salt. Use less or none at all for children. Salt on the outside tricks the tongue into thinking there is salt inside too.
Place the burgers in the fridge to firm up for about 20 minutes.
Heat the frying pan again with another tablespoon of olive oil, and fry the burgers. Aim to turn only once, so you get them nicely browned on each side. This usually takes around 6 minutes a side. Alternatively barbeque the burgers.
Serve in the rolls with little gem lettuce and sliced beef tomatoes.
Gluten-Free – check the Dijon mustard is gluten-free or swap to 1 teaspoon of Colman’s mustard powder.
Water-based fillings like sliced tomato or ketchup can make the bun go soggy, so either spread with a little mayonnaise or butter first to seal, or make sure the lettuce is between it and the bun.