Heat your oven to 220°C/Fan 200°C/Gas 7. Drizzle 2 tbsp oil onto a baking tray. Slide into the oven to warm up. Finely grate the zest from the lemon and put in a large bowl. Squeeze in the juice from half the lemon (keep the other half for later). Grate in the garlic. Stir in the yogurt with 1 tbsp oil. Season.
Spoon half the lemon and garlic yogurt mixture into a serving bowl and put in the fridge. Add the chicken breasts to the rest of the yogurt marinade. Turn a few times to make sure they’re well coated. Set aside to marinate.
Peel the potatoes and cut them into bite-sized chunks. Cut the tomatoes into wedges. Cut the remaining lemon half into thin slices.
Take the baking tray out of the oven and carefully tumble the potatoes and tomatoes onto it. Season and turn a couple of times to coat in the oil. Roast for 40 mins.
Meanwhile, line another baking tray with baking paper. Lift the chicken out of the marinade and lay on the paper. After the potatoes and tomatoes have roasted for 10 mins, slide the chicken into the oven on the shelf below the potatoes. Cook for 30 mins till the chicken is cooked through.
After the potatoes and tomatoes have roasted for 20 mins, toss and add the lemon slices to the tray. When everything is cooked after 40 mins, remove the potato tray from the oven. Add the rocket and toss together. Return to the oven for 2 mins to just char the edges of the rocket.
Serve the chicken with the rocket roasties and tomatoes, and the remaining garlic and lemon yogurt.