Best-Ever Shrimp Étouffée

In a medium or large Dutch oven over medium heat, combine butter and oil. When butter is melted and foaming, add flour and whisk until smooth. Cook, stirring frequently, until roux is golden in color, 5 to 6 minutes.  Add onion, pepper, and celery and cook, stirring frequently, until softened, 4 to 5 minutes. Stir in garlic, green onions, and thyme and cook until fragrant, 1 minute more. Stir in Cajun seasoning and paprika and season with salt and pepper. Add stock 1 cup at a time, whisking well after each addition. Bring to a low simmer and add bay leaf. Cover and cook, stirring occasionally, until sauce has reduced by 1/3 and vegetables are soft, 10 to 12 minutes Stir in shrimp and simmer until shrimp are opaque and cooked through, 4 to 6 minutes. Remove bay leaf. Season with salt and pepper if needed, and spoon over rice. Garnish with green onions before serving.