Preheat the oven to 350°F, lightly grease a 9×13-inch baking dish with nonstick spray, and set it aside. In a large skillet or Dutch oven set over medium heat, cook the ground beef and diced onion, breaking the meat into crumbles, until the beef is browned and the onion has softened—about 10 minutes. Drain any surplus fat. Stir in the minced garlic and sauté for 30 seconds until aromatic, then remove from heat. Mix in the cream of celery soup, beef stock, garlic powder, onion powder, Italian seasoning, paprika, kosher salt, and black pepper until well combined. Fold in the instant rice and frozen vegetables until evenly incorporated, then spread the mixture into the prepared baking dish. Scatter the shredded cheddar cheese across the top, cover tightly with nonstick foil, and bake for 45 minutes or until bubbling and the cheese has melted. Remove the foil and broil briefly to develop a golden, lightly crisped top. Optionally garnish with chopped parsley before serving.