Preheat the oven to 350 degrees. Bring a pot of water to a boil. Add the lasagna and cook according to package directions. When done, remove and rinse under cold water. Drain on a dish towel or paper towels, separating so they don't stick together. Allow to cool a little so you can handle. Brown the sausage in a skillet until there is no trace of pink. Add enough of the jarred sauce so that the meat is coated. Set aside.
In a bowl add the ricotta cheese, Parmesan cheese, parsley and salt and pepper to taste. Add a little salt and pepper and taste, add more salt if needed. Your Parmesan cheese is salty so that is why it is by taste and I always check the seasoning before I add the egg. When you are good with your seasoning, add the egg and combine.
To assemble, take one lasagna strip or sheet and add about 2 tablespoons of the ricotta cheese spreading to cover the entire strip. Then add a little of the Italian sausage mixture, distributing a little over the entire strip or sheet. Lastly, add a little grated Mozzarella cheese to each roll.
Pour a little of the sauce to cover the bottom of a casserole dish large enough to hold the rolls. Starting with the end closest to you, roll each strip and place in the casserole dish, seam side down. Continue until all of the strips or sheets have been rolled.
Pour the remaining sauce over the pasta making sure it's covered from side to side. As I said in the ingredients list, I always have an extra jar on hand just in case I need a little extra.
Cover the casserole with saran wrap, then with foil. Place in the oven and cook for about 30 minutes. Remove from the oven, remove the foil and saran wrap. Add grated mozzarella cheese over the rolls. Allow to sit until the cheese has melted. Serve with additional sauce on the side. Enjoy!