Chop shrimps into small pieces and set aside.
Trim off both ends and peel opo squash, then cut in half lengthwise. Cut in half lengthwise one more time and remove the spongy white part with seeds. After that, cut into 2-inch long chunks.
Bring the stock to a boil in a pot and add opo squash chunks. Cook for 3-5 minutes until the stock is boiling again and opo squash is softened.
Transfer opo squash chunks and the stock to a blender and puree until smooth. You can also use an immersion blender to do this.
Place a pot over medium - medium high heat, add oil and sauté shallots until fragrant. Add shrimp pieces, stir and cook for 30 seconds. Then add crab meat, 1 teaspoon of fish sauce and black pepper, stir and cook for about a minute.
Pour the opo squash puree into the pot. Stir and when it starts to boil, lower heat to a simmer. Add the remaining fish sauce (more or less to taste).
Whisk the cornstarch/tapioca starch with 1 1/2 tablespoons of water and drizzle the mixture slowly into the pot. Gently stir while adding the mixture until the soup is thickened to your liking.
Cook for 1-2 more minutes. Add black pepper and sliced scallions, stir and then transfer to serving bowls. You can add more scallions and cilantro to each bowl.