Preheat the oven to 350 degrees F (175 degrees C).
Heat a large skillet over medium-high heat. Cook and stir ground beef, onion, and garlic in the hot skillet until beef is browned and crumbly, 5 to 7 minutes. Drain and discard grease. Add mushrooms, oregano, hot sauce, salt, and pepper. Simmer for 5 minutes. Add spinach, soup, and sour cream; cook until heated through, about 5 minutes.
Transfer to a casserole dish and cover with mozzarella cheese.
Bake in the preheated oven until cheese is melted, 15 to 20 minutes.
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