Heat the olive oil and butter in a large skillet over medium-low heat. Add the sliced onions and cook, stirring often, until soft and deeply golden, about 25 to 30 minutes. Set aside to cool.

Preheat a smoker to 250°F. In a large mixing bowl, combine the ground beef, ground pork, breadcrumbs, egg, milk, smoked paprika, garlic powder, salt, and black pepper. Mix gently just until combined.

Transfer half of the meat mixture to a sheet of parchment paper and shape it into a rectangle about 9 by 6 inches. Spread the cooled onions over the center, leaving a border around the edges. Sprinkle the cheddar over the onions.

Shape the remaining meat mixture into a layer over the filling. Pinch the edges together to seal, then form the loaf into an even shape. Place it seam-side down on a smoker-safe tray or directly on a clean smoker rack.

Smoke the meatloaf until the center reaches 160°F on an instant-read thermometer, about 2 to 2 ½ hours. Brush the top with half of the BBQ sauce during the final 30 minutes.

Remove the meatloaf from the smoker and let it rest for 10 minutes. Warm the remaining BBQ sauce, slice the meatloaf carefully, and serve with the extra sauce and cornbread.