Because you're not adding flavor to the broth by browning the turkey and cooking it with the liquid, it's important to use good quality chicken or turkey stock. Also don't be afraid for your vegetables to caramelize - all the brown bits from the bottom of the pan will add more flavor to the broth.
In a large stock pot or Dutch oven, heat the oil. Saute the onions, carrots, and scallions over medium-high heat until soft and beginning to brown, about 8 minutes. Add the garlic, ginger, and serrano (if using). Cook for 2 minutes more, until fragrant. Carefully pour in the stock and 2 teaspoons salt. Bring to a simmer, scrapping up any brown bits from the bottom of the pan. Stir in the turkey and pasta. Rapidly simmer until the pasta is al dente (see package directions).
Remove the soup from the heat and stir in the herbs and lime juice. Taste for seasoning and serve piping hot.