Heat a large ovenproof skillet over medium-high heat. Rub chicken thighs with 1 tablespoon mayonnaise; sprinkle evenly with 1/4 teaspoon salt and 1/4 teaspoon pepper. Coat pan with cooking spray. Add chicken, skin side down, to pan; cook 3 minutes. Turn; top evenly with panko and 1 teaspoon thyme. Place pan in oven. Bake at 400°F for 12 minutes or until chicken is done.
Place potatoes in a saucepan filled with water. Bring to a boil; cook 8 minutes or until tender. Place potatoes in a large bowl. Return water to a boil. Add peas; cook 2 minutes. Add spinach; cook 30 seconds. Drain; rinse under cold water. Drain.
Place remaining 2 tablespoons mayonnaise, remaining 1/4 teaspoon salt, remaining 1/4 teaspoon pepper, two-thirds of spinach mixture, vinegar, and garlic in a food processor; process until smooth. Add remaining 1 teaspoon thyme, remaining one-third of spinach mixture, and mayonnaise mixture to potatoes; toss. Serve with chicken thighs.