Heat butter in a medium saucepan over medium heat. When melted, add flour and cook, stirring continuously, until roux turns golden brown and begins to smell nutty, about 3 minutes. Add beer, bring to a high simmer and continue to cook until beer is reduced by half, 4-5 minutes. Add shallot and garlic, and cook, stirring, until soft, about 2 minutes.
Add cream, thyme and bay leaf, and bring to a simmer, stirring often to prevent boiling over. Continue to simmer until sauce thickens considerably, about 10 minutes. Remove from heat and discard thyme sprig and bay leaf.