01Heat the oven to 375 degrees F. Butter a shallow 1 ½ quart ovenproof dish.
02In a small saucepan over medium heat, melt 3 tablespoons butter.
03 In a small bowl, thoroughly combine melted butter with breadcrumbs; set aside.
04Using same saucepan over medium heat, melt 2 remaining tablespoons butter. When butter bubbles, add flour and cook, whisking constantly for approximately 1 minute.
05Slowly pour hot milk into flour-butter mixture, whisking constantly. Continue cooking and stirring until mixture bubbles and becomes thick enough to coat back of a spoon.
06Remove pan from heat. Stir in salt, nutmeg, black pepper, cheddar, and Gruyère. Set cheese sauce aside. Cool 5 minutes, then gently whisk in yogurt.
07 Bring a large saucepan of water to a boil. Add macaroni; cook approximately 3 minutes fewer than indicated on package directions. Pasta will continue to cook in oven. Drain macaroni in colander, then stir macaroni into cheese sauce.
08Pour the mixture into the prepared dish. Sprinkle breadcrumbs over the top. Bake until brown on top, about 30 minutes. Transfer dish to a wire rack to cool for 5 minutes; serve.