Cook noodles according to package instructions, then drain and cool.

In a large bowl, mix noodles with 1 tablespoon of the oil and soy sauce. Set aside.

Roughly chop 8 large basil leaves, then transfer them to a mortar.

Add garlic, chiles and a pinch of salt and grind with a pestle until a paste forms. (If you have no mortar and pestle, mince all ingredients together as finely as possible.)

In a small bowl, mix the brown sugar and the fish sauce together; set aside.

Heat remaining 3 tablespoons oil in a large skillet.

Add garlic-serrano paste and cook, stirring often, for about 1 minute or until fragrant.

Season beef with salt and pepper, then add to skillet and cook until brown, stirring constantly, about 4 minutes.

Add sugar mixture, stir and let cook for 1 to 2 minutes more.

Add noodles and toss until heated through, then add 2 to 3 tablespoons water if it appears sauce is drying up.

Once heated through, remove skillet from the heat and add remaining basil leaves.

Toss again until lightly wilted, then serve immediately, garnished with green onions and lime wedges.