Prepare the Lamb:

1. Lightly coat the lamb shank with flour and season with salt.
2. Heat a medium saucepan over medium-high heat and sear the lamb for 2 minutes on each side. Once browned, remove the shank from the pan.

Cook the Vegetables

1. In the same pan, add the diced onion, carrots, and celery. Sauté until the vegetables are soft and translucent.
2. Add the rosemary sprig and red wine, cooking until the wine reduces by half.

Braise the Lamb:

1. Return the seared lamb shank to the pan. Add the canned tomatoes and chicken stock.
2. Cover the pan tightly with cling wrap and foil. Transfer to a preheated oven and braise at 325°F for 3 hours, until the lamb is fork-tender.

Prepare the Filling:

1. Once the lamb is tender, remove it from the pan and let it cool slightly. Strain the braising liquid and set it aside for the sauce.
2. In a food processor, combine the lamb, vegetables, salt, and pepper. Blend until smooth. Set this mixture aside as the filling for the tortellini.

Reduce the Sauce:

1. Pour the strained braising liquid back into the saucepan and reduce it over medium heat until it thickens by half. This will serve as the sauce.

Make the Tortellini:

1. Cut the store-bought pasta sheets into 3.5 x 3.5-inch squares.
2. Place 1 teaspoon of the lamb filling in the center of each square.
3. Brush the edges of the pasta with egg yolk. Fold the square into a triangle, pressing the edges to seal and remove any air pockets.
4. Shape the tortellini by wrapping the corners around your pinky finger and pressing to seal.
5. Cook and Serve:
6. Cook the tortellini in boiling salted water for 6 minutes, until tender.
7. Toss the cooked tortellini in the reduced sauce and serve with freshly grated Parmesan cheese.
8. Enjoy!