Homemade burgers and oven chips

Method

1. Turn on the oven first, to 220°C fan, with the large metal tray inside, unlined: it needs at least 15 minutes to heat. Scrub the potatoes under running water without peeling and cut into 10 mm sticks. Put in a bowl of cold water and soak for 15 minutes while the oven heats: this removes the surface starch that stops chips browning.
2. While they soak, slice the red onion into very thin rings. Chop 10 g very finely for the sauce; put the rest in a bowl with the cider vinegar, 0.5 g salt and the sugar, mix and leave for 15 minutes: the rings turn pink and soften.
3. Divide the mince into 2 portions of 150 g without kneading. With damp hands, flatten gently into patties 11–12 cm across and 1.5 cm thick, slightly wider than the buns as they shrink, then press a 1 cm deep hollow in the centre with your thumb: this keeps the burger flat as it cooks. Refrigerate on a plate without salting.
4. Drain the potato sticks and dry thoroughly in a clean tea towel: any remaining water would make them boil rather than brown. Wipe the bowl dry and use your hands to coat them in 35 g sunflower oil and 1.5 g salt. Take out the scorching-hot tray, tip on the chips in one go and spread into a single layer with a spatula: they should sizzle. Bake for 15 minutes without disturbing, then turn with a spatula (once brown underneath, they release easily) and cook for another 10–15 minutes: the edges brown, they sound hollow when the tray is shaken, and a knife tip slides in without resistance.
5. During the first 15 minutes in the oven, make the sauce: mix the mayonnaise, ketchup, mustard, 25 g very finely chopped gherkins, chopped onion and 1 pinch paprika. Taste: it should be pink, mild and tangy, and coat a spoon. Refrigerate while the gherkin flavour infuses. Slice the remaining 20 g gherkins and cut the tomato into 4 thick slices; separate and dry the lettuce leaves.
6. When the chips have cooked for 18 minutes, heat the heavy frying pan over a high heat for 2 minutes with 5 g sunflower oil, until just barely smoking. Season the burgers with 2.5 g salt in total, a generous pinch on each side just before putting them in the pan, never earlier, and add pepper. Cook for 3 minutes without touching: a brown crust forms and the burger releases easily. Turn and cook for 3–4 minutes, until clear juices bead on top; insert a thermometer from the side and remove at a core temperature of 68°C: it will rise another 2°C as it rests.
7. Put the burgers on a warm plate and rest for 2 minutes. Meanwhile, discard excess fat from the pan, then toast the cut sides of the buns in it for 1 minute over a medium heat: they should be golden and crisp. Remove the chips and immediately scatter over the fleur de sel and remaining paprika while they are still glossy; if ready before the burgers, turn off the oven and leave them inside with the door ajar.
8. Drain the onions. Spread sauce on both cut sides of each bun. Put lettuce on the base to protect the bread from the juices, then the burger, tomato slices seasoned with 0.5 g salt, gherkin slices and pickled onions. Close, press gently and serve immediately with the chips and remaining sauce for dipping.