Prepare the Giblets and Broth

1. Heat vegetable oil in a medium saucepan over medium-high heat. Add the turkey neck, heart, liver, and gizzards, browning them on all sides.
2. Once browned, remove from pan.
3. Add the onion and celery and saute until softened.
4. Stir in the garlic and thyme and saute for 30 seconds more.
5. Return the giblets to the pan and add enough water to fully submerge the giblets. Bring the water to a boil, then reduce the heat to low. Simmer for about 1 hour, or until the giblets are fully cooked.
6. Strain the broth to separate the giblets and set them aside. Return the broth to the saucepan and refrigerate until you're ready to make the gravy, after roasting the turkey.

Separate the Drippings

1. Let the turkey drippings rest for a minute to allow the fat to rise to the top. Use a large spoon, ladle, or fat separator to skim off most of the fat, leaving the liquid and drippings behind.

Optional Giblet Preparation

1. Once the giblets have cooled, discard the liver if you prefer a milder flavor. Remove the meat from the turkey neck and combine it with the other giblets. Dice the giblet meat into small pieces and set aside for the gravy.

Make the Gravy

1. In a large saucepan, combine the reserved broth, 2 tablespoons of chicken or turkey bouillon base, and 2 cups of the defatted turkey drippings. Bring the mixture to a boil.
2. In a small bowl, whisk together cornstarch and water to create a smooth slurry. Slowly whisk the slurry into the boiling broth.
3. Reduce heat to medium-low and cook, whisking constantly, for 5-8 minutes, or until the gravy thickens. Stir in the diced giblets, if desired.

Adjust the Consistency

1. If the gravy is too thick, gradually add water, ¼ cup at a time, until it reaches your desired consistency. If it's too thin, cook for another 10 minutes or add another cornstarch slurry (1 tablespoon cornstarch mixed with 1 tablespoon water) and whisk until thickened.

Season and Serve

1. Taste and season the gravy with salt and pepper as needed. Serve warm and enjoy!