In a bowl, whisk together olive oil, fresh lemon juice, dried oregano, minced garlic, kosher salt, and black pepper until well combined.

Place the boneless, skinless chicken breasts in the marinade, ensuring they're evenly coated. Cover the bowl with plastic wrap and marinate in the refrigerator for at least 30 minutes, up to 4 hours.

While the chicken marinates, prepare the tzatziki. Grate one cucumber and squeeze out excess moisture using a clean kitchen towel.

In a mixing bowl, combine the grated cucumber with plain Greek yogurt, fresh lemon juice, minced garlic, and chopped dill. Stir until well mixed, then cover and refrigerate.

In a medium saucepan, heat butter or olive oil over medium heat. Add long-grain white rice and toast it for about 1 minute.

Pour in chicken broth and bring to a boil. Once boiling, reduce heat to low, cover, and simmer for 15-20 minutes, or until the rice is tender.

Remove from heat and let the rice rest for 5 minutes before fluffing it gently with a fork.

Preheat your grill to medium-high heat, ensuring the grates are clean.

Lightly oil the grill grates to prevent sticking. Remove the chicken from the marinade and place it on the grill.

Cook for 6-8 minutes on each side or until the internal temperature reaches 165°F (74°C).

Remove from the grill and let the chicken rest for a few minutes before slicing.

To serve, plate a generous portion of herbed rice as the base, then top with slices of grilled chicken. Add a spoonful of tzatziki.