Melt butter in a heavy saucepan over low heat; whisk in flour until smooth. Cook 1 minute, whisking constantly. Gradually whisk in milk; cook over medium heat, whisking constantly, until mixture is thickened and bubbly. Whisk in Swiss and Parmesan cheeses, salt, and red pepper, whisking until cheeses are melted and sauce is smooth.
Excellent - so yummy! I actually made this sauce to top steamed broccoli for company, and everyone loved it! I did leave out the pepper since it was for the broccoli, though. Note - the recipe makes a ton! I should have halved it for my purpose. But it was so good and my hubby and I definitely agreed that we want it again!