In a medium saucepan, combine the rinsed barley with 2 cups water. Bring to a boil, reduce heat to a very low simmer. Cover the pan and cook for 20 minutes or until still a bit al dente. Remove from heat and let stand for 2-3 minutes. Fluff with a fork and let cool.
In a small bowl, whisk together the pesto, lemon juice, red wine vinegar, olive oil, garlic, salt and pepper.
In a skillet, add the white beans (with reserved liquid) and kidney beans, sun-dried tomatoes, and onions. Heat on stove over medium heat until warm, then add in the cooked barley and stir to mix.
Stir the pesto dressing into the skillet of beans. Toss gently to coat everything with pesto dressing.
Add chopped arugula to top beans. Serve while warm.