Preheat the oven to 350°F and grease a 9x13-inch baking dish. In a Dutch oven or large skillet over medium heat, cook the ground beef and diced onion for about 10 minutes, stirring and breaking the meat into crumbles, until browned and the onion softens.

Drain any excess grease. Stir in the minced garlic and cook for 30 seconds. Remove from heat. Add the cream of celery soup and beef stock, then mix in garlic powder, onion powder, Italian seasoning, paprika, salt, and pepper until combined.

Add the instant rice and frozen mixed vegetables. Stir well to evenly combine everything into the creamy mixture.

Transfer the mixture to the prepared baking dish. Spread it evenly and top with shredded sharp cheddar cheese, covering the entire surface.

Cover with nonstick foil and bake for 45 minutes, until bubbly and the cheese is fully melted. Uncover and broil for 2–3 minutes until the cheese is lightly browned. Garnish with parsley if desired and serve warm.