Knead dough lightly 4 to 5 times, kneading in 6 tablespoons Gruyère cheese and 2 tablespoons Parmesan cheese.
Shape dough into an oval loaf; place on a greased baking sheet. Cover and let rise in a warm place (85°), free from drafts, 1 hour or until doubled in bulk.
Combine egg yolk and milk, stirring well; gently brush mixture over loaf. Sprinkle with remaining cheese. Bake at 350° for 45 minutes or until loaf sounds hollow when tapped. Cool on wire rack.