Whisk together the lemon juice, minced garlic, and half of the olive oil in a small bowl.
Place the chicken breast in a shallow dish or bag, coat with the marinade, and let sit for at least 15 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 on the baking sheet.
Roast the broccoli for 15-20 minutes until the edges are slightly crispy and charred.
While the broccoli roasts, rinse the quinoa and combine it with 1/2 cup of water in a small pot.
Bring the quinoa to a boil, then reduce heat to low, cover, and simmer for 12-15 minutes until the water is absorbed.
Heat a grill pan or outdoor grill over medium-high heat and cook the chicken for 6-7 minutes per side until the internal temperature reaches 165°F.
Let the chicken rest for 5 minutes before slicing.
Fluff the quinoa with a fork and serve alongside the grilled chicken and roasted broccoli.