Bring a large pot of salted water to a boil, boil pasta to al dente according to package instructions. Drain pasta and reserve 1/2 cup pasta water.
While pasta is cooking, in a large saucepan (4 quart) or medium pot melt butter over medium heat then add flour, cook 1 minute whisking constantly.
While whisking slowly pour in milk, add in mustard and garlic powder. Increase heat slightly, then bring mixture to a gentle simmer whisking constantly.
Remove saucepan from heat add in the shredded cheeses half at a time, and whisk to melt, while returning to warm heat as needed to melt fully.
Season sauce with salt to taste.
Toss pasta with cheese sauce, thin with pasta water a splash at a time as needed. Serve immediately.
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