TO MAKE THE BRIOCHE: Measure flour into a bowl. In a separate bowl, dissolve yeast in warm water; add 2 tbsp flour from the bowl and whisk until smooth. Cover and let stand in a warm place for 15 to 20 minutes, or until mixture has doubled in volume. Stir sugar and salt into remaining flour in the bowl.
In a large bowl, beat eggs well. Pour yeast mixture into eggs, then add softened butter. Add flour mixture in four batches, mixing after each addition. Turn dough out onto a floured surface and knead for about 10 minutes, or until dough is smooth and sticky.
Transfer dough to a greased bowl. Cover and let stand in a warm place about 1 hour and 30 minutes, or until dough has tripled in size.
TO PREPARE THE SAUSAGE: In a 2 quart ovenproof pan, cover the sausage with stock, wine, and minced scallions. Cover and bake on the middle rack of a 375°F oven for 30 minutes. Remove the sausage and let cool, making sure it is straight and the ends do not curve. Reserve 1 cup of pan liquid for sauce. When sausage is cool enough to handle, remove the sausage casing. Allow sausage meat to cool to room temperature, then roll in flour to coat well.
When brioche dough has risen, remove from the bowl and place on a floured surface. The dough should be soft but firm enough to handle. Sprinkle flour all over the dough and flour a rolling pin. Gently, without pressure, roll the dough to a 10 x 10 inch square. Wrap the sausage in the dough. Place the bundle, seam side down, in a buttered 9 by 5 inch loaf pan. Let brioche rise from about 1 hour, or until it fills most of the pan.
TO MAKE GLAZE: In a small bowl, mix egg yolk and water. Brush glaze over the brioche. Bake brioche on the middle rack of a 400°F oven for 30 minutes, or until brioche is golden brown. Let the loaf cool in the loaf pan for a few minutes, then remove it from the pan and let it cool on a rack for 30 minutes more.
While loaf is cooling, prepare mustard sauce. In a bowl, blend the mustard, butter, and cornstarch until mixture is smooth. Skim any grease off the reserved 1 cup of sausage poaching liquid and pour liquid into a small saucepan.
Add cream and bring to a boil. Whisk the mustard mixture into the boiling liquid. Cook over high heat, stirring constantly, until the sauce thickens slightly. Remove from heat and let cool slightly.
When brioche loaf is cool, slice and serve with warm mustard sauce. Serves 8.