Trim excess fat and cut lamb to 1-inch cubes. Pat them dry with a kitchen towel. (ref note 1). Season with salt and pepper.
Heat oil in a heavy bottom pot with lid to medium heat. Add seasoned lamb pieces in a single layer a little apart ( if the pot is small, brown lamb in 2 batches to avoid overcrowding and prevent it from steaming).
After adding lamb to the pot cook undisturbed for 2-3 minutes, then flip and cook for another 2-3 minutes. Keep aside browned lamb pieces covered.
Making Ragu Base
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.
Pappardelle pasta with Lamb Ragu
Bring a large pot of salted water to a boil. Cook 8-ounce Pappardelle or pasta of your choice to al dente according to package directions. Reserve a cup of pasta cooking water.
Add a couple of ladles of lamb ragu and 1/2 cup of pasta cooking water to a skillet. Let it simmer for 2-3 minutes. Stir in the cooked pasta, about 1/3rd cup of parmesan cheese, and fresh basil. Toss well to coat pasta with the ragu and serve.
Notes
Dry lamb pieces brown and caramelize well. If you have time, you can keep the lamb pieces uncovered in refrigerator for couple of hour for it to dry a bit. If lamb is frozen, keep frozen lamb in a colander, a bowl underneath to collect the liquid, and thaw overnight.
Sauting the sofrito( onion + garlic + celery + carrot) until it is softened and aromatic is an additional step but its worth the extra 5-minutes time.
I personally don't like to taste red wine in the finished dish, hence to avoid that make sure you reduce red wine well until the alcohol smell is almost gone.
To make sofrito, here is a hack- run roughly chopped veggies(onion, celery, carrot) one by one in a food processor until it resembles mince like texture.