Preheat the oven to 350ºF and set a rack in the middle. Line a baking tray with foil or parchment and set aside. If you want, place a baking dish with a few inches of boiled water on a lower shelf which helps keep the meatloaf from cracking.
Add broth to a small bowl or ramekin and sprinkle gelatin over it. Stir to combine and set aside.
Melt the butter in a medium skillet set on medium. Add onions, thyme, salt, and pepper and cook, stirring occasionally, until the onions are soft and translucent but not dark brown, about 8 to 10 minutes. Add anchovy, garlic and tomato paste and cook 3 minutes longer, until garlic is fragrant and anchovy dissolves. Turn off the heat and stir in the soy sauce, Worcestershire sauce and chicken broth-gelatin. Allow the mix to cool slightly.
In a large bowl, combine the ground beef, pork, cooled onion mixture, bread crumbs, and eggs, and mix lightly but throughly with your hands or a fork. Don't squish or the meat loaf will be dense. Shape the mixture into 1 large rectangular loaf (or 2 smaller loaves) on the lined sheet pan. Bake for 1 to 1 1/4 hours (if you made 2 smaller loaves, begin checking after 45 minutes), until the internal temperature reaches 150ºF and the meat loaf is cooked through. (A pan of hot water in the oven, under the meat loaf, will keep the top from cracking.) Remove from the oven and increase the oven temperature to 450°F.
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, about 2 minutes. Remove from the heat.
Use a brush or a spoon to apply a thick layer of glaze to the meatloaf, then return it to the oven and bake for 3 minutes. Glaze again and bake until the glaze is beginning to bubble, about 3 minutes longer. Remove from the oven and allow it to rest for 15 minutes. Slice and serve with any extra glaze, as desired.