In a large bowl, combine the ground beef, breadcrumbs, egg, diced onion, and 1/2 cup BBQ sauce. Mix until evenly blended, then shape into a loaf that will fit in your slow cooker.
Place the meatloaf in the slow cooker and pour the remaining 1/2 cup BBQ sauce over the top. Cover and cook on low for 6 hours until the internal temperature reaches 160°F.
While the meatloaf cooks, prepare the cornbread. In a mixing bowl, stir together the cornmeal, flour, baking powder, milk, melted butter, and remaining egg until just combined. Pour into a greased baking dish and bake at 400°F for 20 minutes until golden on top.
In a large pot, combine the chopped collard greens, chicken broth, minced garlic, salt, and black pepper. Bring to a simmer over medium heat and cook for 25 minutes, stirring occasionally, until the greens are tender.
Let the meatloaf rest for 5 minutes before slicing. Serve slices with wedges of warm cornbread and a scoop of the collard greens.