Make the quick pickles first — they can sit while you prep the rest. Combine 120 ml rice vinegar, 60 ml water, 2 tbsp sugar and 1 tsp kosher salt in a bowl and stir until dissolved. Add the thin matchstick carrots (213 g) and daikon (107 g) and press down so the liquid covers the vegetables. Quick-pickling in an acidic, sweet brine brightens the root vegetables and softens them slightly without making them mushy; it also develops that signature sweet-tang that balances the rich beef. Cover and chill at least 20 minutes (up to 4 hours).

Prepare the lemongrass marinade: in a bowl combine 4 tbsp minced lemongrass, 2 minced garlic cloves, 4 tsp fish sauce or light soy, 1 tbsp granulated sugar, 1 tbsp neutral oil and 1 tsp freshly ground black pepper. Whisk to combine. The lemongrass gives citrusy, floral aromatics while the fish sauce/soy adds umami; a bit of oil helps the marinade coat the thin slices and transfer flavor efficiently.

Trim any excess fat from the flank steak and, if not already thin, slice across the grain into 3–4 mm strips. Toss the steak with the marinade to coat evenly, pressing the lemongrass into the meat. Marinate for 30–60 minutes at room temperature or up to 4 hours in the fridge. Thin slices need less time — the goal is flavor penetration, not tenderizing with acid, so avoid very long marinating with citrus or vinegar here.

Build the flavored mayo: in a small bowl mix 4 tbsp mayonnaise with 1 tsp toasted sesame oil (optional) and a squeeze of lime if you like. The sesame oil adds a nutty, toasty note that elevates the sandwich; mixing it into mayo ensures it spreads easily without overpowering the other elements.

Preheat your grill to high (or heat a heavy cast-iron skillet or grill pan over high heat) and oil the grates or pan with 1 tbsp neutral oil. A very hot surface creates immediate searing — the Maillard crust gives smoky-roasted flavor that contrasts the meat’s tender interior. If using a grill, clean grates and oil them to prevent sticking.

Sear the marinated beef in a single layer (work in batches if needed) for 20–40 seconds per side for thin slices, just until nicely browned and no longer raw. Thin-sliced flank cooks extremely quickly — short cooking at very high heat develops flavor without drying the meat. Transfer to a plate and let rest briefly; resting lets juices redistribute.

Warm and split the baguettes: cut the 2 small baguettes into 4 equal sandwich-length portions and split them open, leaving a hinge on one side. Lightly toast the cut face on the grill or in the pan for 20–30 seconds until slightly crisp. Toasting prevents the bread from getting soggy from the pickles and mayo and adds a satisfying crunch.

Assemble the sandwiches: spread about 1 tbsp of the flavored mayo on the bottom half of each baguette (use less if you prefer). Layer 150 g of the sliced grilled beef per sandwich, a generous handful (about 80 g) of the quick-pickled carrots & daikon, 30 g cucumber slices, and about 6 g fresh cilantro leaves. The order here — mayo, beef, pickles, cucumber, cilantro — balances richness and acidity so each bite has a hit of every flavor.

Finish and serve: squeeze a lime wedge over each sandwich and finish with a crack of black pepper. Serve immediately while the beef is warm and bread still crisp. The lime brightens the whole sandwich and freshly cracked pepper adds an assertive finish.

Storage and make-ahead notes: keep leftover pickles refrigerated up to 5 days in an airtight container. Marinated raw beef can sit up to 4 hours in the fridge before cooking; cooked sliced beef will keep 2–3 days refrigerated and can be gently reheated in a hot pan for 30–60 seconds.