Preheat your grill or grill pan to medium-high heat.
In a small bowl, mix olive oil, lemon juice, minced garlic, salt, and pepper.
Brush the chicken breast evenly with the marinade and let it sit for about 10 minutes.
Grill the chicken for about 6-7 minutes per side or until it reaches an internal temperature of 165°F.
While the chicken is grilling, preheat your oven to 425°F to roast the broccoli.
Toss the broccoli with a little olive oil, salt, and pepper, and spread it on a baking sheet in a single layer.
Roast the broccoli in the oven for 15-20 minutes until tender and slightly crispy on the edges.
Prepare the cooked quinoa if not already done by following package instructions.
To serve, place a serving of quinoa on the plate, top with sliced grilled chicken, and add the roasted broccoli on the side.