Season the chicken breast with sea salt, black pepper, and dried oregano on both sides.
Grill or pan-sear the chicken over medium-high heat for 6-7 minutes per side until the internal temperature reaches 165°F.
Place the egg in a small pot of boiling water for 9 minutes, then immediately transfer to an ice bath to cool before peeling and slicing.
In a small glass jar or bowl, whisk together the extra virgin olive oil, lemon juice, and Dijon mustard until emulsified.
In a large serving bowl, combine the mixed baby greens, sliced cucumber, halved cherry tomatoes, and thinly sliced red onion.
Slice the grilled chicken into strips and dice the avocado, then arrange them over the salad base along with the sliced egg.
Drizzle the lemon-dijon vinaigrette over the top and toss gently to combine all the fresh flavors.