Heat a large, heavy pan (preferably cast-iron) until smoking hot. Rub the beef with the vegetable oil and season well. Sear for around 3 minutes on each side, until browned on the outside and pink in the middle. Allow the meat to cool, then wrap in clingflim and set aside to chill.
Meanwhile, mix half the lime juice and zest with the fish sauce, sugar, lemongrass, ginger and chilli, in a small bowl. Finely slice the beef and add to the bowl with the dressing. Toss to coat.
To assemble the canapés, arrange a little pile of basil, mint and spring onion on each slice of beef (reserving the marinade), then spear with a fork, through both sides, to hold it neatly together.
To make a dipping sauce, combine the reserved marinade with the sweet chilli sauce and remaining lime juice and zest. Serve the dipping sauce with the mint fork and lime wedges to squeeze over, if you like.
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