Split the 12-inch French loaf lengthwise but not all the way through (leave a hinge) and hollow out a little of the soft interior to make room for fillings; discard or save crumbs for another use.

Mix the mayonnaise and Creole mustard in a small bowl until smooth. Spread a generous layer (about half the mixture) on the bottom half of the split loaf and the remaining on the top half when assembling.

Arrange pickle slices down the length of the bottom half where you will stack the proteins so they help hold moisture and add tang.

Fried Protein (Shrimp & Roast Beef)

Season & soak shrimp

Pat the shrimp dry with paper towels. Place shrimp in a bowl with buttermilk and 1/2 tsp of Cajun seasoning; toss to coat and let sit 10 minutes (this tenderizes and helps the coating adhere).

Make dredge

In a shallow bowl or plate combine 1 cup flour, 1/4 cup cornmeal, remaining Cajun seasoning (1 1/2 tsp), smoked paprika, garlic powder, salt and pepper. Mix thoroughly. This will give a crisp, slightly grainy crust.

Heat oil

Pour vegetable oil into a deep skillet or pot to a depth of about 1 to 1.5 inches. Heat over medium-high until oil reaches 350°F (175°C) or a cube of bread browns in about 60 seconds.

Fry shrimp

Remove shrimp from buttermilk, letting excess drip off, then dredge in the flour-cornmeal mixture, pressing to adhere. Fry shrimp in batches for 2 to 3 minutes per side or until golden brown and cooked through. Do not overcrowd. Transfer to a wire rack set over a sheet tray or to paper towels to drain; season lightly with a pinch of salt immediately.

Warm the roast beef

If using deli roast beef, warm slices in a skillet over medium heat for 1–2 minutes with a light sprinkle of Cajun seasoning to reheat. If using leftover cooked roast beef, slice thinly and warm similarly. For an authentically dressed roast beef poboy, you can ladle the finished brown gravy (see next section) over the warm beef just before assembling.

Brown Gravy (optional, for roast beef)

Make gravy

In a small saucepan over medium heat, melt 2 tablespoons beef drippings or butter. Stir in 2 tablespoons flour and cook, stirring, 1–2 minutes until a light roux forms. Slowly whisk in 1 cup beef broth to avoid lumps. Add Worcestershire sauce, salt and pepper. Simmer 3–5 minutes until slightly thickened. Taste and adjust seasoning. Keep warm.

Vegetables & Garnish

Prepare the vegetables: thinly slice tomato and onion, shred the lettuce, and have the hot pepper slices ready. If you prefer looser, pickled onions, toss the sliced onion with 1 tsp vinegar and a pinch of sugar and salt and let sit 5–10 minutes.

Assembly

Assemble poboy

With the bottom half of the loaf already spread with mayo-mustard and lined with pickles, layer warmed roast beef evenly down the length of the loaf. If using gravy, spoon a few tablespoons of warm brown gravy over the roast beef to moisten. Top the roast beef with the fried shrimp in a single layer (you can stack shrimp or mix them with the beef depending on preference).

Add shredded lettuce, tomato slices, and onion (or quick-pickled onion) on top of the proteins. Add optional hot pepper slices if using.

Spread remaining mayo-mustard on the top half of the loaf and place on sandwich. Press gently to compress, then cut into 4 generous portions using a serrated knife. Serve immediately while shrimp are hot and gravy warm.