Rinse tongue well with cold water and place in a deep 6- to 8-quart pot. Add cold water to cover by 3 inches, then add remaining tongue ingredients. Cover pot and bring to a boil. Simmer, partially covered, until tongue is fork-tender, 2 1/2 to 3 hours. fork-tender, 2 1/2 to 3 hours.
Transfer tongue to a cutting board (reserve 1 1/2 cups cooking liquid) and, when cool enough to handle, peel off skin and trim any fat or gristle. Skim off fat from cooking liquid and pour liquid through a paper-towel–lined sieve into a large bowl, discarding solids. Keep tongue warm, covered.
Cook shallot in butter in a 2-quart heavy saucepan over moderate heat, stirring frequently, until softened. Whisk in flour and cook, whisking, 1 minute. Gradually whisk in reserved cooking liquid and cream, then bring to a boil, whisking.
Simmer sauce, whisking, until slightly thickened, 2 to 3 minutes. Whisk in mustard, horseradish, herbs, lemon juice, and salt and pepper to taste.
