Preheat oven to 350°F. Line a baking sheet with parchment paper or grease a loaf pan.
Sweat the onions: In a small pan, heat 1–2 tablespoons of olive oil over medium heat. Add the finely chopped onions and cook gently until softened, about 5–7 minutes. Pour in 4 tablespoons of Stowaway and continue to cook until the beer reduces and the onions are translucent, another 3 minutes. Set the onions aside to cool.
Prepare the meatloaf mixture: In a large mixing bowl, combine the breadcrumbs and beer. Let them sit for a couple of minutes to absorb the liquid. Add the ground beef, sweated onions, garlic, Parmesan cheese, eggs, Worcestershire sauce, Dijon mustard, smoked paprika, salt, and pepper. Pour in the milk and mix everything gently until well combined. Avoid overworking the meat to keep the loaf tender.
Shape the loaf: Shape the meat mixture into a loaf and place it on the prepared baking sheet or press it into a loaf pan.
Prepare the glaze: In a small saucepan, combine the ketchup, brown sugar, Stowaway IPA, Dijon mustard, Worcestershire sauce, and apple cider vinegar. Heat over medium heat, stirring occasionally, until the glaze thickens slightly (about 3–5 minutes).
Cook the meatloaf: Brush a generous layer of glaze over the meatloaf. Bake the meatloaf in the preheated oven for 60–70 minutes, until the internal temperature reaches 160°F. Brush with more glaze halfway through baking.