Preheat your oven to 375°F (190°C). Grease a 9×13-inch baking dish with non-stick spray or butter.
Heat olive oil in a large skillet over medium heat. Add the ground beef and chopped onion. Cook, stirring occasionally, until the beef is browned and the onion is softened, about 7-8 minutes.
Add the taco seasoning to the beef and onion mixture, and follow the packet instructions to add water (usually about 1/4 cup). Stir to combine and cook for another 2 minutes, then remove from heat.
In a separate pot, bring a large pot of salted water to a boil. Add the diced potatoes and cook for about 10-12 minutes, or until the potatoes are fork-tender. Drain well and set aside.
In a mixing bowl, combine the sour cream, salsa, milk, and a pinch of salt and pepper. Stir until smooth.
In the prepared baking dish, layer the cooked potatoes, followed by the beef mixture. Pour the sour cream and salsa mixture evenly over the top.
Sprinkle the shredded cheddar cheese over the casserole and cover with aluminum foil.
Bake in the preheated oven for 20 minutes. After 20 minutes, remove the foil and continue baking for an additional 10 minutes, or until the cheese is melted and bubbly.
Once out of the oven, garnish with black olives (if using) and fresh cilantro. Serve hot.