Melt butter in a large heavy bottomed pot (Dutch oven or stock pot) over medium heat. Add leeks, carrots, celery, garlic, and rosemary and saute until vegetables soften (about 10 min.). Stir in flour and continue to cook until the flour is well incorporated. At this point, stir in the cooked sausage, milk and parmesan cheese. Bring to a boil and then reduce heat and allow to simmer (stirring occasionally) until the gravy thickens (about 10-15 min.). Season with salt and pepper to taste.
Pour thickened gravy into a medium sized casserole dish. Top with scrambled eggs. Cover the casserole dish with the puff pastry dough, crimping the dough around the edge of the dish. Lightly brush the pastry dough with egg wash. Place in preheated oven (400 degrees) and bake for approximately 30 minutes or until the pastry is golden brown.