Melt butter in a medium saucepan over medium-low. Gradually whisk in flour. Cook, whisking constantly, until lightly browned, about 1 minute. Gradually whisk in beer and heavy cream. Cook, whisking constantly, until sauce has thickened and begins to bubble, 3 to 4 minutes. Gradually add Gruyère and Cheddar, whisking constantly, allowing each addition to melt and become incorporated before adding more. Whisk in Worcestershire sauce and dry mustard until smooth. Transfer mixture to a fondue pot; cover and keep warm. Serve immediately.