Bring a pot of water to a boil; prepare a grill or grill pan for medium heat.
Whisk together the yogurt, garlic, lemon juice, 2 teaspoons of the oil and 1/2 teaspoon each salt and pepper in a medium bowl until well combined.
Rub the chicken thighs with the remaining 1 teaspoon oil and sprinkle with 1/4 teaspoon salt and 1/2 teaspoon pepper. Grill until nicely marked and cooked through, 10 to 12 minutes per side. Transfer to a cutting board and let stand for at least 5 minutes.
Add the orzo to the boiling water and cook according to the package directions for al dente, stirring in the peas during the last minute of cooking. Drain the orzo and peas, reserving 1 cup of the cooking liquid. Stir the orzo and peas into the yogurt mixture, along with the goat cheese, 3 tablespoons of the herbs and at least 3/4 cup of the cooking liquid, until well combined. Transfer to a platter.
Thinly slice the chicken and arrange on top of the orzo. Sprinkle with the lemon zest and the remaining 1 tablespoon herbs.
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