• Cook the pasta to al dente according to box directions. 
• Preheat the oven to 400°F

To Make the Filling

• In a large skillet over medium heat add olive oil, onion and mushroom cook for 5 minutes allowing the onion to soften and the mushrooms to release liquid. 
• Add the ground sausage and cook, breaking apart during the process until no longer pink. 
• Transfer to a large bowl. 

To Make the Bechamel Sauce (cream sauce)

• Add the butter to the skillet and melt.
• Dust in the flour.
• Stir until cooked and coated with butter. 
• Slowly add in the milk and whisk until smooth and thickened over medium heat. 
• Remove 3/4 cup of sauce to the side
• Add the sausage mixture to the sauce and stir. 
• Add 1/2 of the total cheese, the 2 egg yolks, and oregano to the sauce and stir. 

Stuffing the Shells

• To a 9x13 casserole dish, use a silicone brush to coat the bottom of the dish with a thin layer of the bechamel sauce.
• Use a spoon or gloved hands to fill the pasta shells with the filling and place them in an even layer in the casserole dish. 
• Top the shells with the reserved sauce, followed by the remaining shredded cheese. 
• Bake for 45 minutes until cheese becomes golden and bubbly.