In a small bowl, whisk together half of the olive oil, lemon juice, minced garlic, and dried oregano to create a marinade.
Place the chicken breast in a shallow dish or reusable bag, pour the marinade over it, and let it sit for at least 15 minutes.
Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper.
Toss the broccoli florets with the remaining olive oil, salt, and pepper, then spread them in a single layer on the baking sheet.
Roast the broccoli for 15-20 minutes until the edges are slightly charred and tender.
While the broccoli roasts, heat a grill or grill pan over medium-high heat.
Grill the chicken for 6-7 minutes per side, or until the internal temperature reaches 165°F (74°C).
Let the chicken rest for 5 minutes before slicing.
Fluff the pre-cooked quinoa and serve it alongside the grilled chicken and roasted broccoli.
Garnish with an extra squeeze of lemon if desired.