Make the Giblet Stock: Add the turkey neck, giblets, and broth to saucepan over medium-high heat. Bring the liquid to a boil, then reduce the heat to low, cover the pot, and simmer for an hour.
giblets and turkey neck, 4 cups chicken broth
Remove the pot from heat, strain the solid pieces out of the broth, and set aside.
Build the Roux: Return the pot to the stovetop over medium heat, and add the butter. Once the butter has melted, add the onion and spices and cook until fragrant (about 2-3 minutes), then whisk in the flour until dissolved.
¼ cup unsalted butter, ½ onion, ½ teaspoon rubbed sage, ½ teaspoon black pepper, ½ teaspoon dried oregano, ½ teaspoon mushroom powder, ½ teaspoon dried basil, ¼ teaspoon ground thyme, 1 bay leaf, sea salt, ¼ cup all-purpose flour
Finish the Gravy: Let the roux develop until the flour starts to turn golden brown (about another three minutes), then stir in the strained stock. Continue cooking for 15-20 minutes, or until the gravy is thickened and bubbly. Taste test, and adjust seasonings as needed.