For the mushroom filling: Heat the olive oil in a pot over medium heat, and then saute the garlic until golden, about 1 minute. Add the mushrooms, onions, some sea salt and pepper and cook until translucent, about 5 minutes. Deglaze the pot with the white wine and porcini powder, and let reduce 30 seconds. Transfer to a food processor and pulse a few times until the vegetables are minced, and then pour into a bowl and fold in the ricotta, parsley and Romano.
For assembling the medallione: Whisk the egg with 2 tablespoons cold water to create an egg wash. Spoon 2 tablespoons of mushroom filling into each pasta square and brush the edges with some egg wash. Place another pasta square on top, pressing the edges together with your fingers, and then seal the square shut with a fork.
Boil the medallione in salted water until al dente, 2 minutes, and then place in a bowl. Ladle some Demi-Glace on top, and garnish with shaved Romano and coarsely cracked pepper.
Preheat the oven to 400 degrees F.
Place the veal bones in a roasting pan and drizzle with the olive oil. Roast until browned, 1 hour to 1 hour 30 minutes, and then transfer to a large pot. Deglaze the roasting pan with water, scraping up the browned bits, and pour into the pot. Add the parsley, peppercorns, carrots, bay leaves, onions, thyme, celery and leeks and top off with cold water. Throw in the tomato paste and mix. Bring to a boil over medium heat, and then turn the heat down to medium-low and let simmer until reduced to 1/4 of the original volume, 10 to 12 hours. Strain the broth, and then ladle through a wire mesh sieve into a saucepan over medium-low heat. Cook to reduce, 2 to 4 hours, skimming off the top every 30 minutes. In the last 5 minutes of cooking, mix in the sage.