Toast the sesame seeds in a hot, oil-less pan, until they are lightly golden.
Remove the green parts of the spring onions (and set aside to use another time).
Trim the lemongrass stalks so that you only have the white parts, and roughly chop them.
Place the white parts of the spring onions, lemongrass and garlic cloves, together with a pinch of salt and pepper, in the bowl of a food processor. Blitz until you have a fine paste, adding some oil to help it along (about 1/2 to 1 tablespoon), if needed.
In a medium mixing bowl, combine the minced beef with the fish sauce, soy sauce, sesame oil, light brown sugar, toasted sesame seeds and the lemongrass paste.
Use your hands to mix everything together.
Cover the bowl with clingfilm and put the bowl in the fridge for about 1 hour for the flavours to develop.
Meanwhile, soak some bamboo or wooden skewers in some cold water for at least 30 minutes. This will stop the skewers from burning when you grill them later.
Shape the beef mixture into long or short sausages and thread them onto the skewers. I used 18cm long bamboo skewers and needed 16 for this amount of beef mixture.
Heat a grill pan or barbecue on medium-high heat and cook the skewers for about 6 minutes, turning them frequently so that they colour and caramelise on all sides.
Serve the lemongrass beef skewers with a baguette filled with some lettuce, pickled shredded carrots and herbs such as mint, coriander (cilantro) and perilla leaves, drizzeld with some Hoisin Dipping Sauce.
Alternatively, serve the lemongrass beef skewers with a vermicelli noodle salad or in rice paper rolls.