Beef Stroganoff

Name: Beef Stroganoff
Ingredients: 4 tbsp. unsalted butter; 2 tbsp. flour; 1 tsp. dry mustard; 1 cup beef stock; 1⁄4 cup sour cream; 1 small yellow onion, thinly sliced; 1 1⁄2 lb. beef tenderloin, cut into 3″ x 1″ x 1/8″ slices; Kosher salt and freshly ground black pepper, to taste; Canola oil, for frying; 4 russet potatoes, peeled and sliced into 1/8″ matchsticks; 2 tbsp. finely chopped parsley