You can easily swap out baby arugula for the spinach if that's what you've got on hand. Slideshow: More Chicken Soup Recipes
In a large heavy pot, heat the butter over medium heat until hot, stir the onions, carrots, celery, 3/4 teaspoon salt, and 1/2 teaspoon pepper and cook, stirring occasionally, until golden, 8 to 10 minutes. Stir in the chicken and stock bring to a gentle boil. Cook the soup, stirring occasionally, until the chicken is cooked through, about 20 minutes. Stir in the pasta and cook until al dente. Stir in the spinach and cook until tender, about 4 minutes. Season the soup with salt and pepper to taste and serve.