Add butter to a large skillet over low-medium heat. When butter has melted, add finely chopped onion and chicken. Season with ½ teaspoon salt and pepper. Increase heat to medium. Stir until chicken has cooked through; 7 minutes. Add blended mixture, stir in milk, season with remaining salt and decrease heat to medium-low. When milk has mixed in completely and mixture begins to simmer, add pasta and carefully stir to coat; 3 minutes.