Wipe out the saucepan. Add the remaining 1 tablespoon of olive oil and the bacon and cook over moderate heat until crisp, 4 minutes. Add the celery and the remaining onion and garlic. Cover and cook over moderately low heat until softened, 7 minutes. Stir in the flour, then gradually whisk in the potato cooking water. Bring to a boil, whisking, and cook until thickened slightly. Add the potatoes and the cream and bring to a simmer. Add the salmon and monkfish and simmer over moderate heat, stirring a few times, until the fish is just cooked, 3 minutes. Add the mussels and clams and pour in their reserved cooking liquid, stopping before you reach the grit at the bottom; stir until heated through. Season with salt and pepper and add the parsley. Serve the chowder in bowls.