Season the chicken breast with salt and pepper and heat a little of the olive oil in a frying pan over medium-high heat. Fry the chicken for about six minutes per side until cooked through and let it rest for a moment under a piece of foil.
Toast the sunflower seeds briefly in a small pan until fragrant. Cut the cucumber into half moons and the avocado into slices, and scatter the mixed greens over a plate.
Arrange the cucumber and avocado over the greens and sprinkle the sunflower seeds over top. Slice the chicken into strips and lay them on top. Mix the remaining olive oil with lemon juice, salt and pepper and drizzle this dressing over the salad.