- 4 large Russet or Kennebec potatoes (3 to 4 pounds), peeled and cut into fries 1/2-inch wide and 1/2-inch thick
- 3 tablespoons unsalted butter
- 5 tablespoons all-purpose flour
- 1/2 cup finely minced onion
- 2 tablespoons minced garlic
- 3 1/2 cups beef stock or low-sodium beef broth
- 3/4 teaspoon freshly ground black pepper
- 1 cup chopped leftover roast beef, preferably homemade
- Salt, to taste
- Peanut oil, for frying
- 4 poboy bread or Italian or French bread, cut into 6-inch pieces
- Mayonnaise, homemade or store-bought
- 6 ounces sliced Swiss cheese
- Hot sauce, for serving (optional)
Rinse the fries well under cool running water until the water runs clear. Then place them in a large bowl and cover with ice water. Refrigerate for at least 30 minutes and up to 2 days.
Meanwhile, make the roast beef gravy: Melt butter in a medium saucepan over medium-high heat and whisk in the flour. Cook, stirring continuously, until a milk chocolate-colored roux is formed, 4-6 minutes. Add the onion and garlic, and cook until soft, 2-4 minutes. Whisk in the beef stock, little by little, and bring the sauce to a boil. Stir in the pepper, reduce the heat to a simmer, and cook until the sauce has reduced to a gravy consistency, 15-20 minutes. Stir in the roast beef and season with salt. Cover and set it aside until you are ready to serve the poboys. The gravy can be made ahead and stored up to 1 week, refrigerated,
Fill a deep 5-quart pot or Dutch oven fitted with a candy or deep-frying thermometer, or an electric deep-fryer, with peanut oil to a depth of 4 inches. (Make sure that you have at least 3 inches of space between the top of the oil and the top of the pot, as the fries will bubble up when they are added.) Heat the oil over medium-low heat until the thermometer registers 325F.
Drain potatoes and wrap them in a clean dishcloth; thoroughly pat them dry. Fry the potatoes in small batches in the hot oil, stirring them occasionally, until they are soft and limp and are beginning to turn blond, 6-8 minutes. Using a slotted spoon or a skimmer, carefully transfer the potatoes to paper towels to drain. Let them rest for at least 10 minutes and up to 2 hours. Turn off the heat.
When you are ready to serve the poboys, set the oven to broil and reheat the frying oil to 350F. (Reheat the gravy if necessary.) Working in batches, return the potatoes to the hot oil and fry again, stirring frequently, until they are golden brown, puffed, and crisp, 2-3 minutes. Transfer the fries to a paper-towel-lined platter and season them with salt.
Halve the bread pieces lengthwise and spread both sides generously with mayonnaise. Place the bottom halves of the bread on a baking sheet and divide the fries evenly among them. Spoon some of the gravy over the fries, and then top with the Swiss cheese. Place the baking sheet in the oven and broil until the cheese is melted, 1-2 minutes. Remove from the oven and top with the remaining bread. Serve immediately, with additional gravy for sopping and with hot sauce if desired.