Pat the chicken dry and pound (if uneven) to an even thickness of about 3/4" — this ensures uniform cooking. Drying the surface with paper towels lets the marinade cling and promotes better browning; even thickness prevents thin edges from overcooking while the center finishes.

Make the simple lemon‑oregano marinade/dressing: whisk together 2 tbsp olive oil, juice of 1 lemon (about 2 tbsp), 1 tsp dried oregano, 1/2 tsp kosher salt and 1/4 tsp freshly ground black pepper in a shallow dish. Reserve one third of this mixture (about 2 tsp) to drizzle at the end. Using the same mixture for both marinade and dressing concentrates the flavors and minimizes dishes.

Add the chicken to the remaining dressing and turn to coat. Marinate at room temperature for 10–15 minutes (or cover and refrigerate up to 2 hours). A short room temp marinade brightens the chicken without denaturing proteins too much; lemon juice seasons and tenderizes subtly but prolonged contact will 'cook' the surface.

Preheat a grill, grill pan or heavy skillet over medium‑high heat until very hot. Oil the grill grates or brush a thin film of oil on the chicken. A properly preheated surface promotes a quick sear and attractive char lines that add flavor through the Maillard reaction.

Grill the chicken 4–5 minutes per side (for 4‑oz breasts) until it reaches 160–165°F (71–74°C) on an instant‑read thermometer. Don’t flip more than once — a single flip gives the best crust. The internal temperature ensures safety while preventing dryness; chicken will carry over cook a few degrees as it rests.

Transfer chicken to a plate and tent loosely with foil; rest 5–7 minutes. Resting lets the juices redistribute so slices are moist rather than releasing fluid onto the plate when cut.

Warm and fluff the quinoa: if the quinoa is chilled, warm it very briefly in a microwave or in a dry skillet over medium for 2–3 minutes, stirring — warm quinoa blends better with the dressing and helps wilt the arugula slightly, creating a more cohesive salad.

Assemble the salad: in a large bowl combine the warm quinoa, arugula, halved cherry tomatoes, diced cucumber and crumbled feta. Toss with the reserved dressing you set aside from the marinade, and season to taste with additional salt and pepper. Tossing while the quinoa is warm helps the leaves soften just a touch and allows the dressing to coat each grain.

Slice the chicken against the grain into 1/4" slices and arrange over the salad. Drizzle any resting juices from the chicken over top and finish with an extra pinch of oregano and a grind of black pepper. Slicing against the grain shortens fibers and makes each bite tender.

Serve immediately so the textures contrast (warm quinoa and chicken, cool crisp cucumber and peppery arugula). If making ahead, keep chicken and dressed salad separate and combine just before serving to preserve texture.