Preheat a medium sauté pan over medium heat and add a long squeeze of olive oil, followed by onions and a pinch of salt. Stir and cook until softened, about 5 minutes. Move onions to a bowl and set aside.
In a food processor, add breadcrumbs. Spin to break down until powdery. Add buttermilk then spin for 10 seconds or until the mixture is hydrated. Add ground meats and process to combine and break down the meat, this should take about 15 seconds.
Transfer the meat mixture to the onion bowl and add eggs, garlic, worcestershire, mustard, poultry seasoning, pepper, and salt. Mix and fold until just combined. Shape into a loaf. I use a loaf pan to shape it, then transfer out onto a foil lined baking tray. Refrigerate for about 10-15 minutes while you make the glaze (recipe below).
Sear the meat loaf under medium-low broiler for 5-10 minutes until beginning to take on color on top. Add a layer of glaze and return to cook under broiler for another 3 minutes.
When glaze gets sticky and begins to take on color, remove loaf from the oven and glaze again. Return the loaf to the oven under the broiler and repeat, rotating as necessary.
Glaze a third/final time then load into a 325°F/160°C to bake for 25-35 minutes or until internal temp reaches 155°F/70°C
Allow the meatloaf to rest for 20 minutes while you make your onion gravy.
After the meatloaf has rested, slice it into 1-1½" (2½-3cm).
Enjoy your meatloaf alongside some mashed potatoes and peas. Smother it all in more glaze and onion gravy.