In a small bowl, whisk together the lemon juice, minced garlic, and half of the olive oil to create a marinade.
Place the chicken breast in a shallow dish, coat with the marinade, and let it sit for at least 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 15-20 minutes until the edges are tender and slightly charred.
While the broccoli roasts, heat a grill pan or cast-iron skillet over medium-high heat.
Grill the chicken for 6-7 minutes per side, or until the internal temperature reaches 165°F.
Allow the chicken to rest for 5 minutes before slicing.
Serve the sliced chicken over the cooked quinoa with the roasted broccoli on the side.