Prepare an outdoor grill, heat to hot, and spray the grid with nonstick cooking oil. Alternatively, heat a grill pan or heavy sauté pan over high heat, add the oil, and swirl to coat the pan.

Bring a large saucepan of lightly salted water to a boil. Cook the orzo until just tender, about 10 minutes. Drain well.

Meanwhile, season the chicken on both sides with salt and pepper. Grill the chicken, turning once, until the juices run clear when the meat is pierced with the tip of a paring knife, about 10 minutes. Alternatively, grill the breasts in a pan over medium-high heat, turning once, until cooked through, about 10 minutes.

Cut the chicken with its skin into 1/4-inch-wide slices. Allow the chicken to come to room temperature if you prefer, or proceed with the warm chicken slices.

In a large bowl, combine the chicken with 1 cup of the pesto, the orzo, and the tomatoes. Season with salt and pepper. In a medium bowl, combine the remaining 1/4 cup of pesto with the lemon juice, then toss with the spinach. Season with salt and pepper.

Divide the spinach mixture among 4 plates. Mound the chicken salad on the spinach, drizzle with additional pesto, and serve.

Make with warm or room temperature chicken. Don't combine all the ingredients until just before serving time.