Adjust oven rack to middle position and heat oven to 325˚ F
Heat oil in Dutch oven over medium-high heat until shimmering. Add sausage and cook until browned, 3 to 5 minutes. Using slotted spoon, transfer sausage to plate lined with paper towel. Add shrimp to pot and cook until shells are lightly browned on both sides, about 1 minute per side. Transfer shrimp to large bowl and refrigerate.
Reduce heat to medium and add onion, celery, bell pepper, and garlic to pot. Cook, stirring occasionally, until vegetables have softened, 5 to 10 minutes. Add rice, tomato paste, salt, and thyme and cook until rice is coated with fat, about 1 minute. Stir in tomatoes, reserved tomato juice, clam juice, bay leaf, and sausage. Place square of aluminum foil directly on surface of rice. Bring to boil, cover pot, transfer to oven, and bake until rice is almost tender and most of liquid is absorbed, about 20 minutes.
Meanwhile, peel shrimp (devein if desired) and discard shells. Remove pot from oven, lift off aluminum foil, and gently stir in peeled shrimp and any accumulated juices. Replace foil and lid, return to oven, and cook until rice is fully tender and shrimp are cooked through, about 5 minutes. Remove from oven, discard foil and bay leaf, and fold in scallions. Serve.