Parboil the Potatoes: Bring a pot of salted water to a boil. Add the diced Yukon Gold potatoes and parboil for exactly 5 minutes until slightly tender but not mushy. Drain thoroughly and pat them completely dry with a paper towel.

Brown the Sausage: Heat your cast-iron skillet over medium-high heat. Add the bulk pork sausage, breaking it apart with a spatula. Cook until perfectly browned and crispy (about 7-8 minutes). Remove the sausage from the skillet using a slotted spoon, leaving the rendered fat in the pan.

Sauté the Leeks: Reduce the heat to medium. Add the freshly cleaned and sliced leeks directly to the hot rendered fat. Sauté for 4-5 minutes until deeply softened and fragrant. Remove the leeks and set them aside with the sausage.

Crisp the Potatoes: If the pan looks dry, add 1 to 2 tablespoons of olive oil. Add the dried, parboiled potatoes to the skillet in an even layer. Let them cook undisturbed for 4-5 minutes to develop a crispy crust (the Maillard reaction). Flip and crisp the other sides for an additional 4 to 5 minutes.

Combine Ingredients: Return the cooked sausage and softened leeks to the skillet with the crispy potatoes. Toss gently to combine and heat everything through for 2-3 minutes. Season with kosher salt and black pepper.

Add Dijon Mustard Finish: In a small bowl, whisk together the Dijon mustard and apple cider vinegar to create a tangy sauce. Remove the skillet from the heat, drizzle the mustard mixture over the hash, and toss gently to coat. Serve immediately.