In a saucepan over medium heat, bring 4 cups of the water to a boil. Using a fine-mesh sieve, plunge shrimp into the hot water and cook for about 1 to 2 minutes.
Remove and allow to cool to touch. Slice lengthwise into halves. Reserve liquid.
In a pot over medium heat, heat oil. Add onions and garlic and cook until softened.
Add ground pork. Cook, breaking apart with the back of a spoon until lightly browned. Drain excess fat except for about 1 tablespoon.
Add fish sauce and cook for about 1 to 2 minutes.
Add atsuete powder and stir until ground pork is evenly colored.
Add reserved liquid and bring to a boil. Lower heat, cover, and simmer for about 6 to10 minutes or until meat is tender and cooked through.
Add shrimp or pork bouillion and stir until dissolved.
In a small bowl, combine the remaining ½ cup water and flour. Stir until well blended and smooth with no lumps.
Gently add to the pot, whisking vigorously to prevent lumps. Continue to cook for about 3 to 5 minutes or until sauce is thickened. Season with salt to taste.
Meanwhile, in a large pot over medium heat, bring about 4 quarts of water to a boil. Add noodles and cook according to package directions until tender but firm. Drain well.
In a large bowl, combine cooked noodles, meat sauce, and ½ cup of the crushed chicharon. Gently toss to evenly distribute.
Transfer to a serving platter. Garnish with the remaining ½ cup crushed chicharon, sliced boiled eggs, toasted garlic, and green onions. Serve hot with calamansi.