Whisk together the lemon juice, minced garlic, and half of the olive oil in a small bowl to create a marinade.
Place the chicken breast in a shallow dish or bag, coat with the marinade, and let sit for 15 to 20 minutes.
Preheat your oven to 400°F 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 to 18 minutes until the edges are tender and slightly charred.
While the broccoli roasts, heat a grill pan or outdoor grill over medium-high heat.
Grill the chicken breast for 6 to 7 minutes per side, or until the internal temperature reaches 165°F.
Let the chicken rest for 3 minutes before slicing.
Serve the sliced chicken over the fluffy cooked quinoa with the roasted broccoli on the side.