01 - Preheat oven to 350°F. Butter a 2-quart baking dish.
02 - Bring a large pot of salted water to a boil. Cook macaroni until just al dente (about 1–2 minutes less than package directions). Drain and set aside.
03 - In a large saucepan over medium heat, melt 3 tbsp butter. Whisk in flour and cook for 1–2 minutes until lightly golden.
04 - Gradually add milk, whisking constantly to avoid lumps, until sauce thickens (about 3–5 minutes). Remove from heat. Stir in cheddar, Gruyère, and mozzarella until melted and smooth. Add Dijon, garlic powder, onion powder, paprika, salt, and pepper. Taste and adjust seasoning.
05 - Add drained macaroni to the cheese sauce, stirring to evenly coat. Pour mixture into the prepared baking dish.
06 - In a small bowl, mix panko with melted butter and parmesan. Sprinkle evenly over the macaroni.
07 - Bake uncovered for 20–25 minutes, until golden and bubbling. Broil for 2–3 minutes for extra crispness if desired.
08 - Let stand 5 minutes. Garnish with parsley before serving, if using.