Combine the garlic, ginger, lemongrass and red chili paste into a blender. Roughly chop the red pepper, onion and garlic. Add it to the blender also. Blitz until a thick paste has formed.
Heat a large saucepan over a medium-high heat, add the coconut or vegetable oil. Fry the paste for a couple of minutes, until fragrant.
Add the chicken broth and coconut milk and bring to a boil, Reduce heat and simmer for five minutes
While the soup base is cooking peel and devein shrimp. Slice in half.
Add the shrimp and fish sauce to the soup base, simmer for an additional five minutes.
While the soup is simmering, cook your noodles (enough for 4 servings). Cooking the noodles according to the package directions. Divide the cooked noodles between four bowls.
Season the soup to taste with a squeeze of lime juice.
Divide between the bowls and top each one with fresh cilantro, slices of fresh chili, and spring onions. Serve immediately.