Heat oven to 350° F.
Melt the butter in a large saucepan over medium heat. Add the onion, celery, carrots, and garlic. Cook until softened, 5 to 7 minutes. Stirring constantly, add the flour and cook for 3 minutes; the mixture will clump. Still stirring constantly, add the broth slowly and cook until a thick, smooth paste forms, 3 minutes. Still stirring, slowly add the milk, then the beer. Cook until the foam subsides and the mixture thickens slightly, about 5 minutes. Add the Cheddar, salt, pepper, sugar, and hot sauce (if using) and simmer over medium-low heat for 20 minutes, stirring occasionally. Let cool for 5 minutes.
Meanwhile, thinly slice the baguette. Arrange on a baking sheet and brush with the oil. Bake until golden, 13 to 15 minutes. Ladle the soup into individual bowls and top with additional grated Cheddar and the baguette slices.