In a large saucepan, heat 1 tablespoon of the olive oil over medium high heat, reserving the other 2 tablespoons. Sear each side of the halibut, leaving each side undisturbed for the first 2 minutes or so to allow the surface of the fish to begin to caramelize and to avoid tearing. Cook until done, about 6 to 8 more minutes, then remove to a utility platter. To the same pan add the shrimp and cook until just pink and remove to a bowl. To the same pan add the scallops and cook until opaque and remove to the bowl with the shrimp. Add the mussels to the pan along with 1/2 cup water. Cook until the mussels open. Remove them and the pan juices to a bowl until they are cool enough to handle. (Discard any mussels that do not open.)
In the same saucepan, heat the other 2 tablespoons of olive oil and butter. Add the celery and onions, sauteing until translucent. Whisk in the flour gradually to make a roux. Add clam juice and fish stock and mix well to integrate flavors. Reduce heat to low and bring to a simmer. Add carrots and parsley and let cook until fork tender.
While the soup is cooking, cut the halibut fillets into 1/2 inch chunks. Then, remove the mussels from their shells and discard shells, but reserving all the liquids in the bowl.
When carrots are softened, remove soup from heat and gently fold in halibut, shrimp, scallops, mussels and liquid in the bowl. Season with salt, pepper and crab boil seasoning, to taste.