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1. Preheat broiler. Coat a 2 1/2- to 3-quart broiler-safe au gratin or baking dish with cooking spray; set aside.** Bring a large pot of lightly salted water to boiling. Add pasta and cook according to package directions. Drain and return to the warm pot.
2. Meanwhile, in a large saucepan melt the 3 tablespoons butter over medium heat. Add onion; cook and stir for 3 minutes. Stir in flour, kosher salt, and pepper. Add milk all at once. Cook, whisking constantly, until thickened and bubbly. Cook and whisk for 2 minutes more. Remove from heat.
3. Add Gouda cheese, cheddar cheese, Swiss cheese, and 1/4 cup of the Parmesan cheese, whisking until cheeses are melted and sauce is smooth. Add the cheese sauce to cooked pasta; stir to coat. Transfer mixture to prepared dish.
4. In a small microwave-safe bowl melt the 1 teaspoon butter. Stir in panko and the remaining 1/4 cup Parmesan cheese. Sprinkle panko mixture over pasta mixture. Broil 5 to 6 inches from the heat about 2 minutes or until top is browned. If desired, garnish with parsley.