Trim the lemongrass to the tender pale cores and finely mince; mince the garlic.
In a bowl, toss the sliced beef with the minced lemongrass, garlic, fish sauce, soy sauce, brown sugar, black pepper and oil until evenly coated. Let marinate 15 to 30 minutes.
Heat a wide pan over high heat until very hot.
Add the beef in a single layer and sear hard, tossing, so the marinade reduces and caramelizes into a glossy, sticky glaze that coats every piece, about 4 to 6 minutes.
Spoon over steamed rice and top with crispy fried shallots.
Serves 2 to 3. Via Benjamin Andersen (@justbenjicooking) on Instagram (reel/DffPIeLIUyE). The reel is video-only with no written recipe, so quantities were reconstructed from the technique shown (slice and mince lemongrass; marinate thin beef with lemongrass, garlic, sugar, fish sauce and soy; sear hard until sticky; serve over rice with crispy shallots).