Rinse the quinoa under cold running water in a fine-mesh sieve.
Combine the quinoa and water in a medium saucepan, bring to the boil over medium-high heat, then reduce to low, cover with a lid and simmer for 15 minutes.
Remove the pan from the heat and leave it, covered, for 5 minutes. Fluff the quinoa with a fork and set aside.
Pat the chicken breasts dry with kitchen paper and season both sides with ½ tsp salt and ¼ tsp black pepper.
Heat the olive oil in a large frying pan over medium-high heat. Add the chicken and cook for 6-8 minutes on each side, or until golden and an instant-read thermometer inserted into the thickest part reads 75°C.
Transfer the chicken to a chopping board and rest for 5 minutes, then cut into bite-sized pieces.
Dice the cucumber, halve the cherry tomatoes and dice the red pepper.
In a small bowl or jar, whisk together the olive oil, fresh lemon juice, dried oregano, Dijon mustard, ¼ tsp salt and a pinch of black pepper until well combined.
Divide the mixed salad leaves between four bowls. Top each with cooked quinoa, diced chicken, cucumber, cherry tomatoes, red pepper and diced ripe avocado.
Drizzle over the lemon and herb dressing just before serving.