Bring a large pot of heavily salted water to a boil. Cook the penne pasta until al dente, about 10-12 minutes. Reserve 1 cup of pasta water, then drain and set aside.

Heat a tablespoon of vegetable oil in a large skillet over medium-high heat. Season shrimp with blackened seasoning and sauté for 1-2 minutes on each side until pink. Set aside.

Reduce heat to high and add a tablespoon of butter. Sear cubed sirloin steak with blackened seasoning for 1-2 minutes until browned. Set aside with shrimp.

In the same skillet, add another tablespoon of butter and cook diced onion for 3-5 minutes until translucent. Stir in minced garlic and cook for another minute until fragrant.

Pour in heavy cream and reserved pasta water, stir to combine. Add Parmesan cheese, extra blackened seasoning, salt, and pepper to taste. Stir until sauce thickens, about 3-5 minutes.

Add drained penne pasta to the skillet and toss to coat in the sauce. Add reserved pasta water if needed to adjust consistency.

Gently fold in cooked shrimp and seared steak, ensuring everything is evenly distributed. Heat through for a minute, then remove from heat.

Serve immediately, garnished with fresh parsley and additional Parmesan if desired.