Stir the ground flaxseed and warm water together in a small bowl. Let the mixture stand for 10 minutes, until thick and gel-like.
Preheat the oven to 350 degrees F and line a rimmed baking sheet with parchment paper. Combine the breadcrumbs and whole milk in a large bowl and let them soak for 5 minutes.
Add the flaxseed mixture, ground beef, onion, garlic, Worcestershire sauce, Dijon mustard, fine salt, Italian seasoning, and black pepper to the soaked breadcrumbs.
Mix gently until the ingredients are evenly distributed without compacting the beef. Shape the mixture into a 9-by-5-inch loaf on the prepared baking sheet.
Stir the ketchup, brown sugar, apple cider vinegar, and Worcestershire sauce together. Spread half of the glaze over the meatloaf.
Bake for 40 minutes, then spread the remaining glaze over the top. Continue baking for 20 to 25 minutes, until the center reaches 160 degrees F.
Rest the meatloaf for 15 minutes before portioning. Resting keeps the juices in the meatloaf and prevents it from crumbling.
