In a large non-stick skillet, heat 1 tsp. oil over medium-high heat; sauté onion until golden; about 5 minutes.
In a medium bowl, combine onion, spinach, zest, feta, ½ tsp. salt and ¼ tsp. pepper; set aside.
Evenly divide onion-spinach mixture over chicken; roll up and secure rolled chicken with toothpicks; sprinkle with remaining ½ tsp. salt and ¼ tsp. pepper.
In same skillet, heat remaining 2 tsp. oil over medium-high heat.
Cook chicken, turning occasionally, until golden brown and cooked through; about 10 minutes.
Remove toothpicks and serve with arugula drizzled with dressing.