Tongue Sandwich with Tarragon or Parsley Sauce Recipe

Name: Tongue Sandwich with Tarragon or Parsley Sauce Recipe
Ingredients: 1 cow or veal tongue, about 1 1/2 pounds; 2 medium onions, peel on; Kosher salt and freshly ground black pepper; 2 tablespoons vegetable or canola oil; 1 slice rustic bread, crust removed, torn into small pieces (about 1/4 cup); 3 1/2 tablespoons red wine vinegar; 2 tablespoons drained capers, finely chopped; 4 anchovy fillets, minced; 2 medium garlic cloves, minced or grated with a microplane grater (about 2 teaspoons); 1 tablespoon Dijon mustard; 1/4 cup olive oil; 3 tablespoons minced tarragon or parsley leaves, or a mix; 1 boiled egg, yolk and white separated; 2 baguettes, or 8 slices of bread; 1/4 cup prepared horseradish; Arugula, watercress, or some kind of bitter green