Spread the lamb out in a single layer and sprinkle with 2 teaspoons of the kosher salt and ½ teaspoon of the black pepper. Let the meat rest at room temperature for about 15 minutes.
Pour the olive oil into a large stockpot and warm over medium heat. When the oil is hot add the lamb in small batches and cook until browned, about 3 to 4 minutes per batch. Don’t crowd the meat or it will steam instead of brown. Transfer the browned lamb to a bowl while you finish cooking the meat. Set the lamb aside while you cook the vegetables.
Add the onions, scallions, celery, and carrots to the stockpot, adding more olive oil if needed. Sauté until the vegetables are tender, about 15 minutes.
While the vegetables cook make the bouquet garni by wrapping the Italian parsley, thyme, bay leaf, garlic cloves, and whole cloves in a large piece of cheesecloth. If you don’t have cheesecloth you can also use a large diffuser. Fold the cheesecloth over the herbs and spices the tie the bundle with kitchen twine to keep everything secure leaving a long piece of twine to keep outside the stockpot. This makes it easy for you to remove the bundle when you’re ready.
Return the lamb along with any juices to the stockpot. Pour in the chicken stock along with the tomatoes and tuck the bouquet garni down into the stew.
Cover the pan and reduce the heat to low. Simmer for one hour until the meat is tender.
While the stew simmers, cook the mushrooms. Melt 2 tablespoons of the butter in a large skillet over medium heat. Add the sliced mushrooms, stirring occasionally until they have released their moisture and are browned. Add ¼ of the kosher salt and ¼ of the pepper.
To finish the stew, melt the remaining 4 tablespoons of butter in a small saucepan over medium heat. Whisk in the flour and cook for about 3 minutes, whisking while the roux cooks. Add about 2 cups of the broth from the stew, whisking until smooth.
Remove the bouquet garni then add the roux along with the lemon juice and zest to the stew, stirring to blend together.
Add the sautéed mushrooms to the stew and serve.