Season the chicken fillet with salt and pepper and cook it in a pan with a splash of oil for approximately six minutes per side until cooked through. Let the meat rest and then slice it into strips.
Arrange the mixed greens on a plate. Slice the avocado and halve the cherry tomatoes; place both on the greens. Toast the pumpkin seeds briefly in a dry pan until they begin to pop.
Whisk the olive oil with the lemon juice, mustard, salt and pepper until smooth to make a dressing. Lay the chicken strips on the salad, drizzle the dressing over it and finally scatter the toasted pumpkin seeds over everything.