Heat your oven to 200ºC/Fan 180ºC/Gas 6. Scrub the sweet potatoes. Chop each potato into 8 chunky wedges. Put in a large bowl. Add 1 tbsp oil and a little salt and pepper. Shake to mix.
Arrange the wedges on a baking tray. Roast for 30 mins till soft and a little charred. Grate or pare the zest from the lemon and set aside. Peel the onion and slice into thin rounds. Put in a bowl with a squeeze of lemon juice.
Put a chicken breast on a chopping board. Carefully slice most of the way through it horizontally (don’t cut all the way), holding it down with the palm of your hand. Open the chicken out like a butterfly. Repeat with the other chicken breast.
Lay a piece of cling film over the chicken. Bash with a rolling pin to flatten them so they are the same thickness. Pop the chicken breasts into a dish. Add 1 tsp ground cumin, 2 tsp oil, a squeeze of lemon juice and some salt and pepper. Turn to coat. Set aside.
Spoon the yogurt into a bowl. Coarsely grate half the cucumber. Use your hands to squeeze as much liquid as you can from the cucumber. Add it to the yogurt. Stir in 1 tsp oil, a squeeze of lemon juice, the lemon zest and a little salt and pepper.
Add the salad leaves to the onion. Season with salt and pepper. Squeeze in the very last of the lemon juice. Toss to mix.
Heat a large griddle or frying pan till smoking hot. Add the chicken. Fry for 6-8 mins, turning once or twice till golden brown on both sides and cooked through. Slice the chicken into finger length pieces. Serve with the salad, the sweet potato wedges and the tzatziki sprinkled with a little more cumin.