Peel and slice the potatoes into ¼-inch thick disks, then cut into ¼-inch thick sticks. Soak the fries in a large bowl of cold water for at least 1 hour or overnight in the fridge to remove excess starch. Drain and rinse the fries, then pat them dry using paper towels. Heat 3 inches of oil in a large deep pot to 300°F. Fry the fries in small batches for 4-5 minutes to cook through. Remove and set aside on a paper towel-lined baking sheet. Increase the oil temperature to 400°F and fry the fries in batches until golden and crispy, about 3-5 minutes. Drain and sprinkle with salt.
In a saucepan, melt butter over medium heat. Whisk in flour until bubbly, then continue whisking for 1 more minute. Slowly pour in the beef stock while whisking constantly. Add ketchup, Worcestershire sauce, salt, and pepper, and bring to a boil. Mix the cornstarch with water to form a slurry. Add the slurry to the gravy and whisk to combine. Simmer for 10-15 minutes until the gravy thickens to coat the back of a spoon. Taste and adjust seasoning if necessary.
To assemble, place the fries on serving plates, top with cheese curds, and pour the hot gravy over to melt the curds. Serve immediately and enjoy!