Drain the juice from the clams and combine the juice with the onions, celery and potatoes in a medium saucepan. Add enough water to barely cover and simmer, covered, over medium heat until the potatoes are tender, about 20 minutes.
In the meantime, melt the butter, blend in the flour and cook, stirring for 1 to 2 minutes. Add the half-and-half, cook and whisk until smooth and thick.
Add the vegetables and drained clams to the flour mixture and heat through. Season to taste with the salt, pepper and sugar. Top with a pat of butter and sprinkle with fresh parsley. Ladle into bowls and serve.
Recipe by Cheery Kitchen at https://cheerykitchen.com/manhattan-clam-chowder/