Bring a pot of water to the boil. Add the potatoes into the boiling water along with a tablespoon of salt and the juice of 1/2 lemon, plus the squeezed lemon carcass. Boil for 10 minutes. Drain and shake well to fluff up and slightly damage the edges of the potatoes.
Add the potatoes to a medium-large roasting tray. Drizzle with 3 tablespoons olive oil, a pinch of salt, and the remaining lemon juice (1 1/2 lemons). Roast for 40 minutes, basting the potatoes with the excess oil every 10 minutes and turning the slices halfway through.
While the potatoes are roasting, tidy up a little, give yourself a little breather, and when they’ve been roasting for 20 minutes, start on the chicken.
Make sure the chicken pieces are dried well with paper towels and season them all over with salt and pepper. On medium-high heat, heat a frying pan large enough to fit the chicken pieces without overcrowding them (if the pan is not big enough, you may have to work in batches). Heat a tablespoon of olive oil and add the chicken skin-side down. Once they are crispy and a dark golden color, about 8 minutes, turn the pieces over and repeat, ensuring they become the same golden color on all sides.
Transfer the chicken to a separate plate. Add another tablespoon of olive oil to the same pan along with the sliced onions. Sauté the onions, scraping any chicken bits from the bottom of the pan along with them until they’re soft and brown, and then remove from the pan.
In a small bowl or glass, combine the Marmite, honey, vinegar, 2 tablespoons water, and a pinch of black pepper. Mix until you have a smooth marinade, making sure the Marmite and honey that sinks to the bottom are incorporated.
Return the cooked chicken to the pan and add half of the marinade mixture. Turn the chicken pieces in the liquid as it bubbles and thickens. It should start to coat the chicken pieces in a generous layer. Once the liquid is thick and caramelized, remove the pan from the heat so it doesn’t burn.
Once the potatoes are crispy and golden, remove the tray from the oven and turn the oven temperature down to 400°F. Top the potatoes with the lemon zest, the onions, and the thyme sprigs, followed by the chicken. Brush the chicken with some more of the marinade and place the dish back in the oven for 5 minutes so the glaze caramelizes and to ensure the chicken cooks through. Repeat this step two more times so the chicken is coated really well and it’s nice and sticky.