Heat your oven to 190°C/Fan 170°C/Gas 5. Warm a medium pan for 1 min over a medium-high heat. Add ½ tbsp olive oil and the diced chicken. Cook for 5 mins till golden, stirring every now and then.
While the chicken cooks, peel and thinly slice the red onion. Peel and crush the garlic. Tip the sliced onion and the crushed garlic into the pan. Season with a pinch of salt and pepper. Cook and stir for 1 min. Tip the pan's contents into an ovenproof dish.
Cut the peaches in half. Scoop out the stones with a teaspoon. Chop the peaches into quarters. Scrub the potatoes and chop them into 1cm-thick slices. Add them both to the dish.
Pour 200ml boiling water into a heatproof jug. Crumble in the stock cube and swirl to dissolve. Stir in 2 tsp of the Dijon mustard (keep the rest for later) and plenty of black pepper. Pour over the chicken and potatoes. Cover with a lid or foil and slide into the oven. Cook for 30 mins.
While the chicken cooks, use a veg peeler to make courgette ribbons. Pop them into a large bowl. When you get to the core, stop peeling and finely chop it. Finely chop half of the tarragon leaves.
Zest and juice the lemon into a small bowl. Add the chopped tarragon leaves. Add in the remaining Dijon mustard and 2 tsp olive oil. Season with a little salt and pepper. Whisk together till you have a creamy dressing. Fold through the courgette ribbons to wilt them slightly.
After the chicken has cooked for 30 mins, remove the foil. Bake for another 10 mins till the potatoes are slightly golden. Most of the liquid in the dish should have been absorbed.
Serve the peachy roast chicken scattered with the remaining tarragon leaves and the courgette ribbon salad on the side.