In a mixing bowl, combine peeled large Argentinian shrimp with lemon juice, olive oil, Cajun seasoning, garlic powder, smoked paprika, and oregano. Marinate for about 10 minutes.

Heat a large skillet over medium-high heat and add olive oil. Add marinated shrimp in a single layer and sauté for 1-2 minutes on each side until they turn pink and opaque. Remove shrimp and set aside.

In the same skillet, melt butter. Add chopped onion and sauté for about 3-4 minutes until soft. Pour in heavy cream, stirring, and mix in the slurry made of flour, cooking for another minute until smooth.

Return cooked shrimp to the skillet and stir until heated through. Add Boursin cheese and parsley, stirring gently until cheese melts and creates a velvety sauce.

Meanwhile, boil salted water in a large pot. Add penne pasta and cook until al dente, about 8-10 minutes. Reserve ½ cup of pasta water before draining.

Add drained pasta to the skillet with the sauce, tossing together, adding reserved pasta water as needed for desired sauciness. Serve hot, garnished with Parmesan and extra parsley.