For the beerbit sauce: Melt the butter in a medium saucepot over medium heat. Add the flour and whisk until blond, about 5 minutes. Add in the milk and beer and whisk. Simmer until thick, about 10 minutes. Then add the Sriracha and jack cheese. Remove from the heat and stir until the cheese has melted. Adjust seasoning with salt and pepper.
For the shoestring potatoes: Preheat the oil to 350 degrees F. Using a mandoline set with the julienne attachment, carefully run the potatoes through, creating thin matchsticks. Place the potatoes into a colander and rinse with water to wash off the excess starch. Then place onto a baking sheet lined with a dish towel and pat as dry as you possibly can.
Carefully place a handful of fries into the hot oil and fry them until crispy, about 2 minutes. Keep the potatoes moving and don't walk away, these small shoestrings go fast! Drain on a paper-towel-lined baking sheet and sprinkle with garlic salt as soon as they come out of the fryer.
For the sandwich build: On the griddled Texas toast, place the ham, then a pile of shoestrings, then ladle the rarebit sauce over the entire thing. Garnish with chopped parsley.