After roasting a turkey, there are drippings left in the bottom of the pan. Depending on the size of your turkey and exactly how you prepare it, your turkey could produce anywhere from 1 to 5+ cups drippings. If you brined your turkey, the drippings will be very salty. It's important to taste the dripping before you start making gravy. If starting with "briny" drippings, use only 1 cup of dripping, or your gravy will be way too salty. Also make sure to use low sodium broth so you have better control over salt content.
Skim off 1/4 cup turkey fat from the drippings and place it in a sauté pan. Set over medium heat and whisk in the flour to create a roux. Add the chopped onion and fresh herbs, and cook until the onions are soft and the roux is golden.
Meanwhile measure out the pan drippings and broth to total 4 cups combined. This is not an exact science, as all drippings are different based on how to prepared your turkey. Be careful not to add too much of the drippings if they are very salty. Also, you want some turkey fat in the mix, but if your drippings are primarily fat, use less drippings and more broth.
Pour the liquid into the roux, whisking to incorporate evenly. Once the mixture is smooth, simmer to allow it to thicken. The thickness of your gravy is entirely up to you. I like mine thick enough so coat a spoon. Once you reach your desired consistency, turn off the heat.
Taste the gravy, then salt and pepper as needed. Gravy will thicken as it cools, so either set it on a warmer or set a lid over the pan, and keep a little extra broth to stir in at the end and loosen the gravy.
When ready to serve, rewarm if needed. Whisk, 1/4-1/2 cup of additional broth into the gravy if needed. Then use a sieve to strain out the onions and herbs as you pour it into a gravy bowl. Serve immediately.