Preheat oven to 325°F and grease a 9×13-inch baking dish.
In a large pot of salted boiling water, cook the macaroni 1 minute less than package directions indicate. Drain and toss with a bit of olive oil to prevent sticking.
While pasta cooks, melt butter in a large saucepan over medium heat. Whisk in flour and cook 1 minute, whisking frequently. Gradually whisk in milk and half and half, whisking constantly, and cook until thickened.
Remove from heat and stir in 3 cups of the cheddar cheese and 1 cup of the Gruyere until melted. Stir in ground mustard, nutmeg, and salt and pepper, and adjust seasoning as needed.
Add cooked pasta and stir to combine. Transfer to baking dish and top with remaining cheeses. Bake until bubbly and lightly golden, 20-30 minutes. Enjoy!