Season the chicken with 2 tablespoons olive oil, oregano or Italian seasoning, salt, and pepper.
Heat a skillet over medium-high heat and cook the chicken until browned and cooked through, about 6–8 minutes. Remove from heat.
Divide cooked rice or quinoa evenly into serving bowls.
Top each bowl with cucumber, cherry tomatoes, red onion, chickpeas, and kalamata olives if using.
Place the cooked chicken on top of the veggies in each bowl.
In a small bowl, whisk together 3 tablespoons olive oil, lemon juice, minced garlic, Dijon mustard, salt, and pepper to make the dressing.
Drizzle the dressing over each bowl, sprinkle with crumbled feta, and serve warm or chilled.