You can soak toasted mustard seeds for this recipe, but you'll get a similar pop of texture simply by using whole grain mustard.
- 2 garlic cloves, finely chopped
- 4 tablespoons unsalted butter
- 2 tablespoons whole grain mustard
- Kosher salt, freshly ground pepper
Preheat oven to 425°. Arrange turkey neck and giblets in a large ovenproof skillet. Roast until very well browned, 35–40 minutes. Remove from oven, add wine, and set over medium heat. Cook, scraping up any browned bits, until wine is reduced by three-quarters, 8–10 minutes. Transfer neck, giblets, and liquid to a large pot; add chicken wings, onion, garlic, and 8 cups water. Bring to a boil, reduce heat, and simmer until liquid is reduced to about 3 cups, about 1 hour. Strain stock through a fine-mesh sieve into a large bowl; discard solids.
Melt butter in a medium saucepan over medium heat, then whisk in flour. Cook, whisking often, until flour begins to smell mildly nutty, about 5 minutes. Whisking vigorously, add warm stock; bring to a simmer. Cook 1 minute; reduce heat to low and whisk in mustard and soy sauce. Season with salt and pepper. Keep warm until ready to serve.
Do Ahead: Gravy can be made 1 day ahead. Cover and chill.