In large pot of boiling salted water, cook pasta according to package directions until al dente. Drain; set aside.
Meanwhile, in large saucepan, melt butter over medium heat; cook garlic and thyme, stirring occasionally, until fragrant, about 2 minutes. Whisk in flour; cook, whisking constantly, for 2 minutes. Pour in milk in slow steady stream, whiskingconstantly until smooth; cook, whisking often, until thickened, about 7 minutes. Whisk in mustard, nutmeg, salt, pepper and cayenne pepper. Stir in Gruyère and Cheddar cheeses until smooth; stir in pasta. Scrape into lightly greased 12-cup (3 L) casserole dish.
: In bowl, combine panko with Parmesan cheese; using fingers, rub in butter until mixture resembles coarse sand. Sprinkle over pasta mixture.
Bake in 400 F (200 C) oven until sauce is bubbly and topping is golden, about 25 minutes. Let stand for 10 minutes before serving.
Tip from the Test Kitchen: The sauce might seem a little runny at first, but it will thicken beautifully when baked with the starchy macaroni.
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