Slice the grilled chicken breasts into 1/2‑cm (1/4 in) thick slices against the grain. Slicing against the grain shortens the muscle fibers and makes the chicken feel tender and juicy even if it’s been stored; even slices also make the salad look composed on the plate.

Halve the cherry tomatoes and place them in a medium bowl. Sprinkle with a pinch of salt and 1/2 tablespoon of the olive oil, then gently toss and let sit for 5–8 minutes (maceration). Macerating draws juices out of the tomatoes and softens them slightly so their natural sweetness and acidity integrate into the dressing instead of remaining dull and raw.

While the tomatoes macerate, thinly slice 1/4 red onion and place the slices in a small bowl of cold water to take some bite out of them for 5 minutes, then drain and pat dry. This step mellows the sharp sulfur notes in raw onion so it complements rather than overpowers the fresh basil and arugula.

Make the dressing: in a small jar or bowl combine the remaining 1 1/2 tablespoons olive oil with 1 tablespoon balsamic vinegar, a generous pinch of salt (about 1/8 tsp), and 1/8 teaspoon freshly ground black pepper. Add optional 1/2 tsp lemon juice if you want extra brightness. Secure the lid and shake, or whisk for 20 seconds to emulsify. Emulsifying coats the salad ingredients evenly — oil alone will separate and leave uneven flavor.

Assemble the salad base on a large shallow bowl or platter: spread the arugula in an even layer (this gives height and a peppery backbone to the salad). Scatter the macerated tomatoes (with their juices), drained onion slices and halved Kalamata olives over the arugula. The arugula’s pepper and the olives’ brine create contrast; placing the tomatoes on top lets their juices dress the greens.

Arrange the sliced chicken over the salad in a fan or loose piles so each serving will have a mix of meat and vegetables. Spoon any tomato juices from the maceration bowl over the chicken — that concentrated tomato oil is flavor gold and helps the chicken marry with the salad.

Drizzle the dressing evenly over the assembled salad and gently toss two or three times to combine, or leave plated and drizzle so the presentation stays clean. Tossing briefly (do not over‑toss) ensures each leaf gets a light coating without wilting the arugula; overstirring crushes tomatoes and makes the greens soggy.

Scatter torn fresh basil leaves on top, season to taste with additional salt and pepper if needed, and serve immediately. Adding basil just before serving preserves its fresh, aromatic lift and keeps the leaves bright green instead of darkening.