Step 1

Combine the chicken stock and buttermilk in a liquid measuring cup and sprinkle the gelatin evenly over the top. Set aside.

Step 2

Place the bread and mushrooms in a food processor and pulse until finely chopped. Transfer to a large bowl and set aside.

Step 3

Add the anchovies, Marmite, soy sauce, paprika, and garlic to the processor bowl and pulse until reduced to a fine paste, scraping down the sides of the bowl as necessary. Add the onion, carrot, and celery and pulse until finely chopped but not pureed.

Step 4

Heat the butter in a 10-inch nonstick skillet over medium-high heat until foaming. Add the chopped vegetable mixture and cook, stirring and tossing frequently, until it is softened and most of the liquid has evaporated, about 5 minutes; the mixture should start to darken a bit. Stir in the buttermilk mixture, bring to a simmer, and cook until reduced by half, about 10 minutes. Transfer to the bowl with the mushrooms and bread, stir thoroughly to combine, and let stand until cool enough to handle, about 10 minutes.

Step 5

Add the meat mixture to the bowl, along with the eggs, cheese, parsley, 1 tablespoon salt, and 1 teaspoon pepper. With clean hands, mix gently until everything is thoroughly combined and homogeneous; it will be fairly loose. Pull off a teaspoon-sized portion of the mixture, place it on a microwave-safe plate, and microwave it on high power until cooked through, about 15 seconds. Taste the cooked piece for seasoning and add more salt and/or pepper as desired.

Step 6

Transfer the mixture to a 9- by 5-inch loaf pan, being sure that no air bubbles get trapped underneath. (You may have some extra mix, depending on the capacity of your pan; this can be cooked in a ramekin or free-form next to the loaf.) Tear off a sheet of heavy-duty aluminum foil large enough to line a rimmed baking sheet and use it to tightly cover the meatloaf, crimping it around the edges of the pan. Refrigerate the meatloaf while the oven preheats. (The meatloaf can be refriger- ated for up to 2 days.)

Step 7

Adjust an oven rack to the lower-middle position and preheat the oven to 350°F. When the oven is hot, remove the meatloaf from the refrigerator and, without removing the foil cover, carefully invert it onto the rimmed baking sheet. Loosen the foil and spread it out, leaving the pan on top of the meatloaf (see note). Fold up the edges of the foil to trap the liquid that escapes from the meatloaf while baking. Bake until just beginning to set (the top should feel firm to the touch), about 30 minutes.

Step 8

Use a thin metal spatula to lift an edge of the inverted loaf pan, jiggling it until it slides off the meatloaf easily, and use oven mitts or a folded kitchen towel to remove the pan, leaving the meatloaf on the center of the foil. Return to the oven and bake until the center of the meatloaf registers 140°F on an instant-read thermometer, about 40 minutes longer. There will be quite a bit of exuded juices; this is OK. Remove from the oven and let rest for 15 minutes. Increase the oven temperature to 500°F.

Step 9

Meanwhile, Make the Glaze: Combine the ketchup, brown sugar, vinegar, and pepper in a small saucepan and cook over medium-high heat, whisking occasionally, until the sugar is melted and the mixture is homogeneous, about 2 minutes. Remove from the heat.

Step 10

Use a brush to apply some glaze to the meatloaf in a thin, even layer, then return it to the oven and bake for 3 minutes. Glaze again and bake for 3 minutes longer. Glaze one more time and bake until the glaze is beginning to bubble and is a deep burnished brown, about 4 minutes longer. Remove from the oven and allow to rest for 15 minutes. Slice and serve with any extra glaze and mustard or ketchup as desired.