Heat the oil in a medium saucepan set over low heat. Add the jalapeno, if using, and cook 3-4 minutes, until just softened. Whisk in the flour to form a thick paste; cook 1 minute. Slowly whisk in the beer; bring to a simmer over medium heat.
Slowly whisk in the cheese a little at a time, allowing it to fully melt before adding more. When all of the cheese has melted, stir in the pumpkin puree and mustard powder. Cook until heated throughout. Season to taste with salt and pepper.