You can use any kind of seafood you like in this sweet-and-spicy soup, which is special enough for a dinner party.
Blanch salmon in boiling water for 10 to 20 seconds. Lift out; drain in a colander. Blanch scallops for 10 seconds; drain.
Heat oil in a small stockpot over medium heat. Add chile paste and garlic. Cook until fragrant, about 10 seconds. Add stock, cover, and bring to a boil. Reduce heat to medium; stir in salmon and scallops, tomatoes, pineapple, onions, fish sauce, sugar, salt, and lime juice. Cook just until seafood is cooked through, about 3 to 5 minutes. Remove from heat, stir in scallions, bean sprouts, basil, and cilantro. Divide among 8 soup bowls, garnish with Fried Shallots, if desired, and serve immediately.