2 lbs. medium potatoes (red yukon gold, russet), peeled2 tablespoons unsalted butter1/2 cup chopped onion3 tablespoons all-purpose flour2 cups milk (i used skim)1 teaspoon dry mustardpinch cayenne pepper1/2 teaspoon salt2 cups (8 oz.) sargentoâ® fine cut shredded colby-jack cheese, di