Separate the meatloaf drippings. After removing the meatloaf from the pan, carefully pour the drippings into a heatproof measuring cup or glass jar. Let the drippings sit for several minutes so the fat rises to the top.
Measure the fat and drippings. Skim off 4 tablespoons of fat from the drippings. If you don’t have a full 4 tablespoons, add enough unsalted butter to equal 4 tablespoons total. Reserve the remaining meatloaf juices separately. Measure the drippings and add enough beef broth to equal 1½ cups total.
Make the roux. Add the reserved meatloaf fat and/or butter to a medium saucepan over medium heat. Once hot, whisk in the flour. Cook, whisking frequently, for 1–2 minutes, until the roux is lightly golden and no longer smells like raw flour.
Add the beer. Slowly pour the Festbier into the roux while whisking constantly. The mixture will thicken quickly at first. Continue whisking until smooth.
Add the meatloaf juices and broth. Gradually whisk in the 1½ cups of reserved meatloaf juices and beef broth. Continue whisking until the gravy is smooth.
Simmer the gravy. Bring the gravy to a gentle simmer, then reduce the heat to medium-low. Cook for 6–8 minutes, stirring occasionally, until the gravy has thickened and the beer flavor has mellowed. It should be thick enough to coat the back of a spoon but still be easily pourable.
Add the cream and seasonings. Reduce the heat to low. Stir in the heavy cream, ½ teaspoon Dijon mustard, and a pinch of nutmeg. Taste and add additional Dijon if desired. Season with black pepper and salt as needed. For an extra-smooth gravy, blend briefly with an immersion blender, keeping the blender head fully submerged to avoid splattering.
Serve. Serve generously over sliced meatloaf and mashed potatoes.