Finely chop 1 onion and half the cilantro and parsley leaves. Put into a large bowl with the ground beef, 1 teaspoon of the chili flakes, the egg, and plenty of salt and pepper.
Work the ground beef and other ingredients together using your hands, until everything is well combined. Shape the meat into 8 patties, making sure they are nice and flat, then chill in the fridge for at least 30 minutes.* The burgers can be made up to a day ahead.
Make the chimichurri salsa. Halve the tomatoes, then scoop out the seeds with a spoon and discard. Finely chop the flesh. Crush the garlic and finely chop the remaining onion and herbs, including their stems if they are soft.
Mix the tomatoes with the garlic, onion, and herbs. Stir in the remaining chili flakes, vinegar, sugar, oregano, oil, and Tabasco. Season with salt and pepper. Although the salsa contains lots of fresh herbs, it holds up well and can be made an hour or so ahead. Keep it chilled.
Before you begin cooking, check that your charcoal is glowing white hot, or your gas grill is preheated to 400ºF. Cook the burgers for 2 minutes on each side for medium rare, or 3-4 minutes on each side, for medium or well done. Split the buns and toast on the grill rack if you like. Top the bottom halves of the buns with a good spoon of mayonnaise and some salad greens, Add the burgers and spoonfuls of the chimichurri salsa to serve.
*Taste a bit--The beauty of making your own burgers is that you can cook a little piece to check the seasoning. Pinch off a little meat, heat a skillet or frying pan and cook for a couple of minutes. Taste, then add more seasoning if you need to. I find that burgers normally need a surprisingly large amount.
If it rains--Char-grill, broil or sauté the burgers until golden and juicy.
Recipe courtesy of Fresh & Easy: What to Cook and How to Cook It by Jane Hornby/Phaidon Press, 2012.