In a small mixing bowl, whisk together the 3 tbsp olive oil, 2 cloves garlic, 1 tsp dried oregano, 2 tbsp fresh lemon juice from about 1 lemon, ½ tsp kosher salt, and ¼ tsp black pepper. Place the 1 ½ lb boneless skinless chicken breasts pounded to even thickness in a shallow dish or Ziploc bag, pour the marinade over them, and turn to coat. Let marinate for at least 15 minutes at room temperature (or up to 4 hours in the fridge).

Preheat a grill or grill pan over medium-high heat. Remove the 1 ½ lb boneless skinless chicken breasts pounded to even thickness from the marinade and grill for 5–7 minutes per side, until the internal temperature reaches 165°F and grill marks form. Transfer to a cutting board, rest for 5 minutes, then slice against the grain.

In a small mixing bowl or quart jar, whisk together the 4 tbsp olive oil extra-virgin, 2 tbsp red wine vinegar, 2 tbsp fresh lemon juice from about 1 lemon, 1 tsp dried oregano, ½ tsp kosher salt, and ¼ tsp black pepper until emulsified. Taste and adjust seasoning as needed.

In a large mixing bowl, combine the chopped large romaine lettuce, 2 medium roma tomatoes wedges, sliced whole English cucumber halved lengthwise and sliced, ½ whole red onion, 1 whole green bell pepper seeded and sliced into strips strips, and ½ cup kalamata olives. Drizzle with the dressing and toss gently to coat.

Divide the dressed salad among four plates or bowls. Top each with sliced grilled 1 ½ lb boneless skinless chicken breasts pounded to even thickness and crumbled 6 oz feta cheese. Serve immediately.