Toss chicken cubes with olive oil, 1 teaspoon salt, black pepper, garlic powder, and oregano in a large mixing bowl until evenly coated.
Heat a grill pan or cast-iron skillet over medium-high heat. Add chicken in a single layer and cook for 8 to 10 minutes, turning occasionally, until the internal temperature reaches 165 degrees Fahrenheit.
While chicken cooks, steam broccoli florets in a steamer basket over boiling water for 4 to 5 minutes until tender-crisp and vibrant green.
Prepare the sauce by whisking together Greek yogurt, mayonnaise, minced garlic, lemon juice, and 0.5 teaspoon salt in a small bowl until smooth.
Divide cooked brown rice or quinoa evenly among four meal prep containers.
Top each container with equal portions of grilled chicken and steamed broccoli.
Drizzle the creamy garlic sauce over each bowl or portion into small containers for later use.