Whisk together the lemon juice, half of the olive oil, and garlic powder in a small bowl to create a marinade.
Place the chicken breast in a shallow dish, coat with the marinade, and let it rest for at least 10 minutes.
Preheat your oven to 400°F and line a baking sheet with parchment paper.
Toss the broccoli florets with the remaining olive oil and a pinch of sea salt, then spread them evenly on the baking sheet.
Roast the broccoli for 15-20 minutes until the edges are tender and charred.
While the broccoli roasts, heat a grill pan or outdoor grill to medium-high heat.
Grill the chicken for 6-7 minutes per side, or until the internal temperature reaches 165°F.
Let the chicken rest for 5 minutes before slicing it into strips.
Serve the sliced chicken over the warm quinoa alongside the roasted broccoli.