Pat the chicken breasts dry and place them in a shallow dish or container.
Prepare the marinade by combining the Greek yoghurt, garlic, turmeric, paprika, black pepper and salt in a medium bowl. Whisk until smooth and well combined.
Add the chicken to the marinade, turning to coat evenly. Cover and refrigerate for at least 2 hours, or preferably overnight for maximum flavour.
Cook the chicken using one of the following methods:Sous vide: Preheat a sous vide water bath to 63°C (145°F). Vacuum seal the marinated chicken and cook for 1½ hours. Remove from the bag and sear in a hot pan or on a grill for 1–2 minutes per side until lightly golden.Grill or oven: Preheat the grill or oven to 200°C (400°F). Cook the chicken for 20–25 minutes, turning halfway through if grilling, until the thickest part reaches an internal temperature of 74°C (165°F). Transfer to a cutting board and rest for 5 minutes before slicing.
Arrange the baby gem lettuce on a serving platter. Top with the avocado, cherry tomatoes, sweetcorn and red onion.
To make the dressing, whisk together the olive oil, lemon juice and garlic in a small bowl. Drizzle over the salad and season with salt and freshly ground black pepper.
Slice the chicken and arrange it over the salad. Finish with a sprinkle of nigella seeds and serve immediately.