In a small bowl or a zip-top bag, combine one tablespoon of olive oil, lemon juice, minced garlic, dried oregano, salt, and pepper. Place the chicken breasts in the mixture and coat them thoroughly. Let them sit for at least 10 minutes to absorb the flavors while you prepare the other ingredients.
Heat a grill pan or outdoor grill over medium-high heat. Place the marinated chicken on the grill and cook for about 6 to 7 minutes per side, or until the internal temperature reaches 165°F (74°C). Once cooked, remove the chicken from the heat and let it rest for 5 minutes before slicing it into strips.
While the chicken is resting, divide your cooked quinoa or brown rice into two serving bowls. If the grains are cold, you can lightly steam them or microwave them for 30 seconds to refresh the texture.
Place a handful of spinach or kale on top of the grains. If you prefer a softer texture for the kale, you can massage it with a tiny drop of olive oil and a pinch of salt before adding it to the bowl.
Arrange the grilled chicken slices, cherry tomatoes, diced cucumber, and avocado slices over the greens. Drizzle the remaining olive oil or your favorite light dressing over the top. Add feta cheese or a dollop of hummus if you desire an extra layer of creaminess.