Prep the chicken: Pat the 4 chicken breasts dry and, if not already even, place each between plastic wrap and gently pound to an even ~1/2-inch (1.2 cm) thickness. Even thickness lets the chicken cook at the same rate so you don't get dry edges and an undercooked center.

Make the lemon‑herb marinade: In a bowl whisk together 2 tbsp of the olive oil (reserve the rest for dressing and finishing), the zest of 2 lemons, 4 tbsp lemon juice, the 2 minced garlic cloves, 2 tbsp chopped mint, 1 tsp salt and 1/2 tsp freshly ground black pepper. Add the chicken and turn to coat. Marinate at room temperature for 15–30 minutes or refrigerate up to 2 hours. Acid and oil in the marinade both flavor the chicken — acid (lemon) brightens and lightly tenderizes, while oil and salt help the meat retain moisture during grilling.

Rinse and toast the quinoa: Place the 1 1/3 cups dry quinoa in a fine mesh sieve and rinse thoroughly under cold water to remove the bitter saponins and ensure a clean, delicate flavor. Drain well. Heat a medium saucepan over medium heat, add 1 tsp olive oil and the drained quinoa, and toast, stirring, for 1–2 minutes until it becomes fragrant and slightly nutty. Toasting develops flavor and helps the grains stay distinct when cooked.

Cook the quinoa: Add 2 2/3 cups water and a pinch of salt to the toasted quinoa, bring to a gentle boil, then reduce to low, cover and simmer for 12–15 minutes until the water is absorbed and the germ rings have unfurled. Turn off the heat and let it steam, covered, 5 minutes more — this rest lets the steam finish the grains so they stay fluffy rather than gummy.

While the quinoa cooks, heat a grill or heavy grill pan to medium‑high. Lightly brush the grill/grill pan with the optional tsp of butter or a tiny splash of oil to prevent sticking. Remove chicken from the marinade and let excess drip off; discard any leftover marinade. Pat the chicken dry if very wet — surface moisture will steam the meat instead of searing it. Brush the chicken lightly with a small amount of the reserved olive oil.

Grill the chicken: Place chicken on the hot grill and cook 4–5 minutes per side (depends on thickness) until you get a golden char and an internal temperature of 165°F (74°C). Searing at moderate‑high heat creates Maillard browning for savory flavor; finish by letting the chicken rest 5 minutes before slicing to allow juices to redistribute.

Blanch the peas: When the quinoa has about 2 minutes left, bring a small pot of water to a simmer or use the same pot after removing quinoa. Add the peas and blanch 60–90 seconds just until tender and bright green (frozen peas only need brief warming). Immediately transfer to an ice bath or rinse under cold water to stop cooking and preserve color and texture. Quick blanching keeps peas sweet and prevents them from becoming mealy.

Fluff and cool the quinoa: Fluff the cooked quinoa with a fork and let it cool 3–5 minutes so it isn't steaming hot when mixed with the salad ingredients (excess heat can wilt the arugula). Stir in the blanched peas while warm to temper them and distribute heat evenly.

Assemble the dressing: In a small bowl whisk together the remaining 2 tbsp olive oil, 2 tbsp lemon juice (reserve 2 tbsp from original 4 tbsp if you prefer more acid in dressing), 2 tbsp chopped mint, a pinch of salt and pepper and a little lemon zest. Taste and adjust — the dressing should taste bright, not heavy. The oil binds flavors and carries aroma; mint and lemon keep it fresh.

Finish the salad: In a large bowl combine the fluffed quinoa and peas, crumbled feta, the arugula, and the dressing. Toss gently — the arugula adds peppery contrast and wilt slightly against the warm quinoa, which softens its bite and melds the salad.

Serve: Slice the grilled chicken against the grain into 1/4‑inch slices and arrange over or alongside the quinoa salad. Sprinkle extra mint, a pinch of lemon zest and a final drizzle of olive oil if desired. Serve immediately so the chicken is warm and the salad is bright and slightly warm or room temperature.