In a bowl, stir the lemon juice into 2 quarts of cold water. Cut off the gnarly parts of the celery root (it is too much trouble to pare all the nooks and crannies). Using a sharp knife, peel the celery root. Cut in half and remove any soft spongy center parts. Remove any skin still in the crevices with the tip of your knife. Cut into 1/2-inch pieces and place in the lemon water.

Bring a large pot of lightly salted water to a boil over high heat. Drain the celery root and add it to the boiling water. Return to a boil and reduce the heat to medium. Cook the celery root until tender when pierced with the tip of a knife, about 7 mnutes. Drain and rinse under cold running water. Transfer to a bowl, cover and set aside. (The celery root can be  prepared up to 4 hours ahead and stored at room temperature.)

In a Dutch oven or soup kettle, melt the butter over medium-low heat. Add the red pepper and leeks, cover, and cook, stirring occasionally, until the red pepper softens, about 5 to 7 minutes. (The leek/red pepper can be prepared up to 4 hours in advance and stored at room temperature. Reheat over low heat before proceeding.)

Pour the half-and-half and oyster juices into the Dutch oven and increase heat to high. Cook until small bubbbles appear around the edges, about 5 minutes. Add the oysters and celery root and cook just until the oysters curl around the edges and are heated through, about 2 minutes. Season with salt and pepper and serve. Garnish with parsley.