Season the lamb shoulder and oyster blade all over with salt and pepper.
In a high-sided braising pot, heat the olive oil over medium-high heat and brown the shoulder and oyster blade on all sides. Take your time – this will build great flavor in the finished dish. Preheat the oven to 280°F.
Remove the meat, discard the spent oil, replace with fresh oil and brown the merguez sausages.
Add the onion and garlic and cook for a few more minutes. Add the red wine, stirring with a wooden spoon to loosen any browned bits from the bottom of the pan. Stir in the tomato purée, then return the browned lamb shoulder and oyster blade to the pot along with the bay leaves.
Transfer the pot to the oven and cook for 2½ hours, or until the shoulder meat moves freely from the bone.
Remove from the oven and cool to room temperature, uncovered, then refrigerate.
Scrape the congealed fat from the top and discard. Reheat the sauce over medium heat.
Remove all of the meat from the pot. Wearing gloves if needed, remove the meat from the bones and cut into 1-inch pieces. Slice the merguez sausages into ½-inch coins and return all of the meat to the pot.
Simmer over medium-low heat for another 20-30 minutes to reduce the sauce. Taste and adjust the seasoning with salt and pepper as needed.
Meanwhile, make the frico by sprinkling a spoonful of grated Parmesan into a nonstick pan in a straight line. Cook until the cheese melts, releases its moisture and fat, and becomes golden and crisp. Carefully transfer to a cooling rack and allow to cool completely.
Prepare the parsley cream by mixing mascarpone with finely chopped parsley and seasoning with salt to taste.
Bring a large pot of salted water to a boil and cook the rigatoni until al dente.
To serve, for every 4-5 oz of cooked rigatoni, heat 1 heaping cup of lamb sugo with ½ tbsp butter. Toss the pasta in the sauce until coated, then finish with Parmesan and parsley to taste. Garnish with a frico and a spoonful of parsley cream.
