Butterfly each of the chicken breasts, making 4 thinner pieces of chicken. Working with one piece at a time, place the chicken between two pieces of plastic wrap and flatten with a mallet until the chicken is an even thickness.

Place the pretzels in a food processor and process until you have crumbs. You want the crumbs to mostly be fine, but you still will have a few bigger chunks.

In a shallow bowl or dish, combine the flour, garlic powder, and onion powder. Season to taste with salt and pepper.

In another dish, whisk the egg.

Place the crushed pretzel in a third dish.

Preheat the oven to the lowest setting.

Add about ½” of oil in a heavy skillet. Heat until about 350ºF.

Working with one piece of chicken at a time, dip the chicken into the flour mixture, coating completely. Then dip the chicken into the egg. Last, dip the chicken into the pretzels. Use your hands, if needed, to press the pretzel crumbs into the chicken.

Place the chicken in the hot oil. You may need to do this in two batches to avoid overcrowding the skillet. Cook for 3-4 minutes, then flip and cook until cooked through, about 3 more minutes. Remove the chicken to a cooling rack over a baking tray. Place in the oven to keep warm. Repeat with the remaining chicken, if desired.

To make the sauce, melt the butter in a saucepan. Whisk in the flour and cook for 1 minute, whisking continuously. Whisk in the milk, then the mustard. Cook for a few minutes, until slightly thickened. Add in the cheese, one handful at a time, whisking until melted. Taste and season with salt and pepper, if needed.

Serve the chicken topped with the mustard cheddar sauce. Top with minced parsley, if desired.