Cook the vegetables: Melt butter in a heavy-bottomed, large pot over medium heat. Add the onions and cook until softened, about 3 minutes. Add the celery and cook, stirring for 8-10 minutes. Add the potatoes. Add enough water to the pot to just cover the vegetables with water. Bring to a boil, then reduce heat and simmer until potatoes are tender but not too soft, about 10-15 minutes (depending on the size of the dice). Remove from heat.
Poaching Liquid: In a second pot, heat the 4 cups of water with a bay leaf and the 1/4 onion until just before boiling. Maintain temperature just below boiling and add the sole and scallops. Cover and cook just until seafood is opaque and tender. Remove fish and and scallops to a bowl, reserving the poaching liquid for your stock. Discard the bay leaf and quartered onion.
Soup: Melt the butter in a saucepan over medium heat. Add the flour, stirring to make a smooth paste. Cook for about 1 minute. Gradually whisk in the reserved poaching liquid until your have a smooth mixture. Cook until it begins to thicken. Return the pot with the potatoes to medium heat. Slowly add the the butter/flour/poaching liquid mixture to the pot with the potatoes. Add the fish and scallops. Continue cooking and stirring until chowder thickens. Add the cream and stir until heated through, but do not boil. Season with a pinch of nutmeg and a generous amount of salt and pepper, to taste.
Heavy cream is recommended for the creamiest chowder. If you don't have or don't want to use heavy cream, you can use a light cream. Simply mix it with 1 Tbsp cornstarch before adding to the soup.