Pound or score the chicken. For even cooking, lightly pound the chicken breasts to an even thickness (about 3/4 inch) or make a few shallow diagonal cuts.
This helps the marinade soak in and prevents dry spots.
Mix the marinade. In a bowl, whisk together the zest and juice of 1 lemon, 3 tablespoons olive oil, 2 minced garlic cloves, 1 tablespoon chopped fresh thyme, 2 tablespoons chopped parsley, 1 teaspoon salt, 1/2 teaspoon black pepper, and a pinch of red pepper flakes. Add 1 teaspoon honey if your lemons are very tart.
Marinate the chicken. Add the chicken to the marinade and coat well. Cover and chill for at least 30 minutes (ideal: 2–4 hours).
Don’t go much longer than 8 hours—acid can start to toughen the meat.
Rinse and cook the quinoa. Rinse 1 cup quinoa in a fine-mesh strainer to remove bitterness. In a pot, bring 2 cups broth (or water) to a boil with a pinch of salt. Stir in quinoa, reduce heat to low, cover, and simmer for 15 minutes.
Turn off heat and let steam, covered, for 5 more minutes.
Fluff and season the quinoa. Uncover and fluff with a fork. Drizzle with 1 tablespoon olive oil and a squeeze of lemon. Taste and season with salt and pepper.
Fold in chopped parsley or baby spinach if using.
Heat the grill or pan. Preheat an outdoor grill to medium-high or heat a grill pan/cast-iron skillet over medium-high. Lightly oil the grates or pan to prevent sticking.
Grill the chicken. Remove chicken from the marinade and let excess drip off. Grill 4–6 minutes per side, depending on thickness, until the internal temperature reaches 165°F (74°C).
If using a pan, cook in batches to avoid steaming.
Rest and finish. Transfer to a plate and rest for 5 minutes. Squeeze the second lemon over the chicken and scatter fresh herbs on top for a bright finish.
Plate the meal. Spoon a bed of quinoa onto each plate. Slice the chicken and arrange over the top.
Add a drizzle of olive oil, extra pepper, and a lemon wedge on the side.
Optional extras. For a fresh bite, top with halved cherry tomatoes, chopped cucumber, crumbled feta, or a quick yogurt sauce (yogurt + lemon + garlic + salt).