Heat the olive oil in a skillet over medium-low heat. Add the sliced onion and cook, stirring occasionally, until soft and deeply golden, about 25 to 30 minutes. Set aside to cool slightly.
Prepare a smoker for cooking at 250°F. In a large bowl, combine the ground beef, mashed black beans, breadcrumbs, egg, milk, adobo sauce, cumin, salt, and black pepper. Add the cooled onions and mix gently just until combined.
Shape the mixture into a compact loaf and place it on a piece of foil with the edges turned up slightly to catch drippings. Set it in the smoker and cook until the center reaches 160°F, about 1 1/2 to 2 hours. Check the temperature with an instant-read thermometer.
While the meatloaf smokes, rinse the rice under cold water. Combine it with the water in a saucepan and bring to a boil. Cover, reduce the heat to low, and simmer until the rice is tender and the water is absorbed, about 15 to 18 minutes. Remove from the heat and let it sit, covered, for 5 minutes.
Fluff the rice with a fork, then stir in the lime juice and chopped cilantro. Let the meatloaf rest for 10 minutes before slicing, then serve it with the cilantro-lime rice.