• 2 Tbsp. unsalted butter
• 2 Tbsp. extra-virgin olive oil
• 2 large garlic cloves, finely chopped
• 2 oz. finely grated Parmesan (about ½ cup)
• ½ tsp. Diamond Crystal or ¼ tsp. plus ⅛ tsp. Morton kosher salt
• 2 tsp. Diamond Crystal or 1 tsp. Morton kosher salt, plus more
• 1 lb. dried elbow macaroni
• ½ cup (1 stick) unsalted butter, plus more for baking dish
• 6 Tbsp. all-purpose flour
• ½ tsp. ground white pepper
• 1½ Tbsp. mustard powder
• 16 oz. coarsely grated extra-sharp cheddar cheese (about 6 cups)
• 2 oz. grated Parmesan (about ½ cup)