In a heavy saucepan, cook the celery in the butter over moderately low heat, stirring, for 3 minutes, or until it begins to soften. Stir in the celery seeds and the flour and cook the mixture, stirring, for 3 minutes.
Add the milk in a stream, whisking, and salt and pepper, to taste and simmer the sauce, stirring, until it is smooth and thickened. Stir in the zest, the lemon juice, and the Brussels sprouts and simmer the mixture, stirring occasionally, until the Brussels sprouts are heated through. Transfer the creamed Brussels sprouts to a heated dish.