In a 4-cup measuring cup pour in the tomatoes and add enough water to bring the total to 4 cups. Set aside.
Heat a 12" skillet over medium heat and add the olive oil. Add the onion and 1/2 teaspoon salt and cook the onions until translucent. About 5 minutes. Add the garlic and cook 30 seconds.
Add the sausage and cook, crumbling and breaking it up until no pink remains. If your meat is very fatty, drain off what you can. Add the Italian seasoning and stir well.
Add the broken lasagna noodles to the skillet. Add the diced tomatoes and tomato sauce. Cover and bring to a simmer (not a rolling boil) and cook stirring occasionally 20 minutes. Your sauce should look watery after 15 minutes of cooking. If yours is very dry, add 1/2 cup water and stir it in.
After the 20 minutes, remove the lid and stir in the parmesan cheese. Add the ricotta in drops and slice the fresh mozzarella. Lay slices on top and you can either cover with the lid and place the skillet off the heat for 5 minutes for the cheese to soften or place it under the broiler just to melt the cheese.