Heat oil in a large nonstick skillet over medium-high heat. Add bell pepper, onion and mushrooms and cook for 5 minutes.
Stir in sliced ribeye, salt, pepper and garlic. Cook for 2-3 minutes until beef starts to brown.
Whisk together flour, beef stock, heavy cream and Worcestershire sauce. Pour mixture into skillet.
Add rigatoni and bring to a boil. Reduce heat to medium, cover and simmer for 15 minutes, stirring often.
Remove lid and mix in 1⁄2 cup shredded mozzarella until melted.
Top with remaining cheese, replace lid and cook 5 more minutes to melt cheese.