In a small bowl, whisk together lemon juice, minced garlic, olive oil, salt, and pepper to create the marinade.
Place the chicken breast in a resealable bag or shallow dish and pour the marinade over it. Let it marinate in the refrigerator for at least 30 minutes.
Preheat the grill (or grill pan) to medium-high heat.
Grill the chicken breast for about 6-7 minutes per side or until fully cooked and juices run clear.
Meanwhile, preheat the oven to 425°F. Toss broccoli florets with a little olive oil, salt, and pepper, and spread them on a baking sheet.
Roast the broccoli in the oven for 15-20 minutes until edges are slightly crispy.
If needed, reheat or prepare quinoa according to package instructions (cooked quinoa should be about 1 cup).
Plate the quinoa, top with sliced grilled chicken breast, and add the roasted broccoli on the side.
Finish with an optional drizzle of extra lemon juice for brightness and serve warm.