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1
Soak mussels, clams, and shrimp in separate bowls of salted cold water for 1 hour. Drain. Clean and devein shrimp, reserving heads and shells.
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2
Combine shrimp heads and shells, 2 quarts water, tomato, quartered onion, carrot, and celery in a large pot; bring to a boil. Reduce heat, cover, and simmer until flavors combine, 40 minutes to 1 hour. Season with salt and pepper.
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3
Strain stock through a fine-mesh sieve, discarding solids. Keep warm over low heat.
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4
Heat 2 tablespoons olive oil in a large pot over medium heat. Add chopped onion; cook and stir until softened, about 5 minutes. Stir in rice until translucent and coated with oil, 2 to 3 minutes. Increase heat to medium-high; pour in 1 cup white wine. Simmer until alcohol has evaporated, about 3 minutes.
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5
Reduce heat to medium and stir in 1 ladle of stock until rice has absorbed the liquid. Repeat with remaining stock, stirring after each addition, until rice is creamy and almost tender, about 15 minutes.
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6
Heat 1 tablespoon olive oil in a large saucepan over medium heat. Add 1 clove garlic and cook until golden, 1 to 2 minutes. Add mussels and 1/2 cup white wine; cover and cook until mussels open, about 5 minutes. Remove from heat; discard garlic and any mussels that did not open. Remove shells and keep mussels warm. Strain cooking liquid through a fine-mesh sieve into a bowl.
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7
Heat 1 tablespoon olive oil in the same saucepan over medium heat. Add 1 clove garlic and cook until golden, 1 to 2 minutes. Add clams and 1/2 cup white wine; cover and cook until opened, about 5 minutes. Remove from heat; discard garlic and any clams that did not open. Remove shells and keep clams warm. Strain cooking liquid through a fine-mesh sieve into a bowl.
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8
Heat remaining olive oil in the same saucepan. Add 1 clove garlic and shrimp; cook and stir until shrimp turn pink, about 2 minutes. Remove from heat and discard garlic.
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9
Pour some of the mussel and clam cooking water over the rice; cook for 3 to 5 minutes more. Remove rice from heat. Fold in mussels, clams, and shrimp. Sprinkle parsley on top. Let stand before serving, about 5 minutes.
