Heat the oven: Heat the oven to 200 degrees C (180 degrees C fan). Line a large roasting tray with baking paper.
Make the glaze: Whisk the honey, mustard, olive oil, lemon juice, and garlic in a small bowl with a pinch of salt and pepper. Taste it: it should be sweet first, then tangy. Add more mustard if you want it sharper, or a little more honey if you want it milder.
Prepare the tray: Toss the potatoes with a drizzle of olive oil and salt and spread them in a single layer in the tray. Pat the chicken dry, season with salt and pepper, and nestle the pieces among the potatoes, skin side up. Brush about half the glaze over the chicken.
Roast: Roast for 35 to 40 minutes, brushing the remaining glaze over the chicken halfway through. The chicken is ready when the skin is sticky and golden and the meat is cooked through, and the potatoes are crisp at the edges and soft inside. If the glaze starts to darken too much, cover the tray loosely with foil for the last 10 minutes.
Rest and serve: Rest the chicken and potatoes for 5 minutes, spoon the sticky pan juices over, scatter over the parsley, and serve straight from the tray with the potatoes.