In a large mixing bowl, combine the shallot, garlic, granulated sugar, fish sauce, neutral oil, sesame oil, soy sauce, dried lemongrass, and a generous amount of freshly ground black pepper. Whisk the mixture thoroughly to ensure the ingredients are well incorporated.

Add the beef slices to the marinade, tossing them gently to coat every piece evenly. Cover the bowl with plastic wrap and refrigerate for at least 1 hour, allowing the flavors to meld. For deeper flavor, let the beef marinate for up to 12 hours.

Place the grated carrots and daikon radish into a fine-mesh strainer positioned over a medium-sized heatproof bowl. Sprinkle the vegetables with 2 teaspoons of granulated sugar and 1 teaspoon of kosher salt. Massage the mixture with your hands for about 2 minutes, squeezing to release as much liquid as possible. Discard the drained liquid and transfer the vegetables to a clean bowl.

In a small saucepan, combine the vinegar, water, and the remaining 1/2 cup of granulated sugar. Heat the mixture over medium-high heat, stirring frequently, until the sugar has fully dissolved and the liquid begins to steam, about 2 minutes.

Pour the hot brine over the carrot and daikon mixture, ensuring the vegetables are fully submerged. Cover the bowl and let it rest for at least 1 hour to pickle.

Make Ahead Tip: Store the pickled vegetables in an airtight container, ensuring they remain fully submerged in the brine. They can be refrigerated for up to 2 weeks.

Preheat the oven to 375°F. Position racks in the center and upper third of the oven. Line a baking sheet with parchment paper or foil, then place a wire rack on top.

Remove the beef slices from the marinade, allowing any excess liquid to drip off. Arrange the beef in a single layer on the wire rack. Bake on the center rack until the internal temperature of the meat reaches 150°F, approximately 14 to 16 minutes.

Switch the oven to broil mode and transfer the baking sheet to the upper rack. Broil the beef for 5 to 7 minutes, flipping halfway through, until both sides are beautifully charred.

Spread 1 tablespoon of mayonnaise on the bottom half of each baguette. Layer the baguette with the broiled beef, a generous helping of pickled vegetables, a cucumber spear, a few slices of jalapeño, and fresh cilantro sprigs. Add a dash of Maggi seasoning or soy sauce for an extra burst of flavor.

Make Ahead Tip: The broiled beef can be prepared up to 2 days in advance. Store it in an airtight container and refrigerate until ready to use.

Serve immediately and enjoy the perfect balance of savory, tangy, and fresh flavors.