You can easily swap out baby arugula for the spinach if that's what you've got on hand. Slideshow: More Turkey Soup Recipes
In a large heavy pot, heat the oil over medium heat until hot, stir the onion, garlic, 3/4 teaspoon salt, and 1/2 teaspoon pepper and cook, stirring occasionally, until golden, about 6 minutes. Add the turkey breasts and stock bring to a gentle boil. Cook the soup until the turkey is cooked through, 25 to 30 minutes. Stir in the spinach and cook until tender, about 4 minutes. Season the soup with salt and pepper to taste and serve.