Step 1In a small saucepan, melt the butter over medium heat. Whisk in the flour and cook until the flour is completely incorporated to form a thick, pale roux. Remove from heat and set aside.
Step 2In a 2-quart pot, bring the ale to a simmer. Whisk in the roux a little at a time until the mixture is the consistency of peanut butter; you may not use all of the roux. Whisk in the heavy cream, Worcestershire and dry mustard. Bring to a simmer again and fold in the cheddar and Stilton cheeses and continue cooking until the mixture is smooth. Remove from heat.
Step 3Transfer the mixture to a shallow oven-proof dish or ramekins and place under the broiler until the top is lightly browned. Serve immediately with the toasted baguette slices.