Prep the chicken: pat the two chicken breasts dry with paper towel and lightly score the surface (1–2 shallow slashes) — this helps the marinade penetrate and promotes even cooking. Season both sides with 1/2 tsp salt and 1/4 tsp black pepper.

Make the marinade/dressing base: in a small bowl whisk together 1 tbsp olive oil, 1 tbsp lemon juice, the minced garlic, dried oregano and Dijon mustard (if using). Reserve half the mixture (about 1–1 1/2 tsp) to use as the salad dressing; pour the rest over the chicken. Marinating lightly for 10–15 minutes at room temperature will add flavor and help the chicken brown. (Why: acid and oil carry flavor into the chicken and tenderize slightly; reserving some dressing keeps the salad bright when served.)

Warm the quinoa (optional): if your cooked quinoa is cold, spread it on a plate and microwave for 30–45 seconds or warm in a skillet for 1–2 minutes. Fluff with a fork and let it cool slightly. (Why: gently warming lifts the grains so they remain fluffy and absorb dressing better; cold quinoa is fine too if you want a chilled salad.)

Preheat your grill or heavy skillet: heat a grill pan or outdoor grill to medium-high (about 400°F / 200°C) or warm a heavy skillet over medium-high heat and add a thin film of oil. You want good heat so the chicken develops a golden crust quickly without overcooking inside. (Why: high heat creates Maillard browning for savory, caramelized flavor.)

Grill or sear the chicken: place the marinated chicken on the hot grill or skillet. Cook for about 5–6 minutes per side (depending on thickness) until well browned and an instant-read thermometer inserted into the thickest part reads 74°C / 165°F. Avoid flipping repeatedly; one turn lets a proper crust form. (Why: proper searing locks in juices and creates flavor; use a thermometer to prevent drying out.)

Rest and slice the chicken: transfer the chicken to a cutting board and let rest for 5 minutes before slicing thinly against the grain. Resting redistributes juices so the meat stays moist when sliced. (Why: cutting immediately causes juices to run out, producing drier meat.)

Assemble the salad base: in a large bowl combine the mixed greens, halved cherry tomatoes, sliced cucumber, and the warmed/fluffed quinoa. Sprinkle the crumbled feta evenly over the top. (Why: layering warm quinoa with cool vegetables keeps textures contrasting and the salad from becoming wilted.)

Make and toss with dressing: whisk the reserved marinade with the remaining 1 tbsp olive oil, 1 tbsp lemon juice, and a pinch of salt and pepper to taste. Pour the dressing over the salad and toss gently until everything is lightly coated. (Why: emulsifying the dressing with mustard or vigorous whisking binds oil and acid so the dressing clings to leaves and grains.)

Plate and finish: divide the salad across two plates, fan the sliced chicken breast over each portion, and finish with a final drizzle of extra lemon juice if you like, plus an extra crack of black pepper. Serve immediately. (Why: plating with chicken on top shows off the char and makes it easy to get a balanced bite.)