Peel and devein shrimp, place them in a bowl, and set aside. Save the heads, legs, and skins.

Add shrimp heads, legs, and skins into a separate pot. Add 4 cups water to that pot and bring to a boil over medium heat. Cover and simmer for 20 to 30 minutes. Filter the shrimp broth with a colander or sieve as you pour it into a large bowl. Set aside. Discard the shrimp heads, legs, and skins.

Combine onion, garlic, ginger, and lemon grass in a food processor. Blend until everything is mixed well with a smooth texture. Add the mixture and curry powder into a bowl and mix with a fork to form a thin paste.

Heat 1 tablespoon oil in a pot over medium heat. Add the paste from step 3 and stir constantly until fragrant, 3 to 4 minutes. Add shrimp soup from step 2. Bring the soup to a boil. Turn to low heat, cover, and simmer for 20 minutes. Add more water if the soup gets too thick.

Add shrimp to the curry soup and cook for 2 to 3 minutes, until shrimp is fully curled and turns pink. Scoop shrimp out with a slotted spoon and place in a small bowl.

Lightly season chicken with salt. Add cornstarch to chicken and mix well with a fork. Heat the remaining 1 tablespoon oil in a nonstick skillet. When oil is hot, spread chicken in the skillet and saute over medium high heat until the bottom side turns golden brown. Flip to cook the other side until chicken is cooked through. Turn off heat and transfer chicken to a bowl.

Bring a large pot of water to boil. Add bean sprouts and boil for about 30 seconds, then transfer sprouts to a plate with a colander or strainer.

In the same pot, add noodles and cook according to the instructions. Drain noodles with strainer and set aside.

Add coconut milk to the curry soup. When the soup comes to a boil, add fried tofu and simmer for another 3 to 4 minutes.

Add soup and noodles evenly into three large bowls. Top with tofu, shrimp, chicken, cucumber and bean sprouts. Garnish with cilantro.

Serve immediately.