In the same pot(with all the goodness from brown caramelized bits), add onion. Saute for a minutes until it is translucent.
Add in carrot and celery. Saute for another 2- minutes. Towards the end add minced garlic and cook for 1 minute until it smells good.
Stir in tomato paste and saute on low taking care not to burn.
Deglaze pot by adding red wine followed by tomatoes. Scrap the bottom of the pan to remove any stuck brown bits. ( If not scraped well, the brown bits might burn while slow cooking the ragu later).
Let the wine reduce by half and you can't smell alcohol anymore.
Add beef stock, bay leaf, rosemary, thyme and seasoning. Give everything a good stir and let it cook for 5 minutes.
Add browned lamb and any liquid in the container back to the pot. Cover and let it simmer on stove top for 1 hour 30 minutes or until the lamb is fork tender.
To braise in the oven - Close the pot with lid and place it in a pre-heated oven @350 F for up to 2 hours- 2.5 hours or until lamb is fork tender.
Take out the required amount to toss some pasta, rest let it cool, and freeze in small portions.