For the cheese sauce: In a medium saucepan over medium heat, melt the butter with the flour. Cook, whisking continuously, until the roux starts to smell toasty but has not taken on any color yet, about 5 minutes. Carefully whisk in the milk until there are no lumps. Add the beer, mustard powder, salt and nutmeg. Bring to a boil, whisking, then cook until thick enough to coat a spoon, 5 to 7 minutes. Add the grated cheese and cream cheese and whisk until melted. Turn the heat to very low and keep warm.
For the cheesy pickle nachos: Score the brats and char in a grill pan heated over medium-high for a few minutes on each side or in a saute pan under the broiler. (This can also be done on the grill outside.) Remove from the heat and slice on a hard bias. Set aside and keep warm.
Dump the bag of chips into a serving dish. Top with the warm cheese sauce, sliced brats, pickle chips and chopped dill. Drizzle on the sriracha and ranch dressing and serve immediately.