Preheat oven to 325 degres F (160 degrees C).
Season Lamb Shoulder (5 pound) generously with Salt and Pepper (to taste).
Heat Vegetable Oil (2 tablespoon) in a large braiser or dutch oven over medium high heat. Sear roast until browned on all sides.
Remove roast to a plate and add White Mushroom (1 pound), sautéing until browned and starting to give off their liquid. Add Garlic (5 clove) and continue to cook for 1-2 minutes, stirring often.
Add Sherry Wine (2 1/2 cup) and reduce over medium-high heat for 2 minutes. Stir in Chicken Broth (3 cup) and heat to a simmer.
Add roast back into pan, along with Bay Leaf (1) and Fresh Thyme (5 sprig). Cover and cook in oven for 2 hours.
Turn roast over and replace lid, continue to cook for an additional 2 hours.
Remove from oven and carefully lift roast out of pan onto a plate. Cover with foil and allow to rest while completing sauce. Remove bay leaf and thyme sprigs and discard.
Skim any excess fat from liquid and simmer over high heat to reduce liquid to about 2 cups, approximately 15 minutes.
Whisk together Heavy Cream (3/4 cup) and Corn Starch (1 teaspoon). Whisk into sauce along with the Fresh Thyme Leaves (2 teaspoon) and bring back to a simmer. If sauce is not thick enough, continue to reduce for 2-3 minutes until the desired consistency is reached. Adjust sauce with Salt and Pepper (to taste).
Remove butcher string from roast and slice against the grain into 1/4-inch slices. Arrange slices in desired serving dish and ladle with sauce. Serve and enjoy!