Peel and devein shrimp (reserve shells and heads), season generously with Creole seasoning, and refrigerate while you prep.

Place shrimp shells and heads, quartered onion, smashed garlic cloves, and bay leaf in a large saucepan. Cover with 3 quarts cold water. Bring to a simmer over medium heat, adjust heat, and simmer 30 minutes. Strain stock through a fine-mesh strainer, pressing against solids with back of a spoon to release any trapped liquid; discard solids and rinse saucepan. Measure 8 cups broth into saucepan and keep warm until needed (any additional broth may be frozen for another use).

Heat oil in a Dutch oven over medium-high heat. Add onions; cook onions, stirring constantly, until translucent, 5-8 minutes. Make a well in the center and add garlic and tomato paste. Cook, stirring, until tomato paste begins to dissolve in oil, 1-2 minutes, then stir the pan through. Add bell pepper, celery, and 2 teaspoons Creole seasoning; sauté until vegetables are soft, about 5 minutes.

Add rice and cook, stirring occasionally, until rice is translucent, about 5 minutes. Add warm shrimp broth; stir well. Taste and adjust seasoning. Bring mixture to a low boil, cover, reduce heat to low, and simmer 30 minutes undisturbed.

Remove lid (do not stir). Evenly distribute raw shrimp over hot cooked rice in a single layer. Top with sliced green onions (do not stir). Replace lid and cook over low heat for 3 minutes. Remove from heat, do not uncover, and let sit 15 minutes. Gently stir, taste, and adjust seasoning.