Pour olive oil into a saucepan and heat on medium.
Add chopped onion to oil and cook until onions are translucent in color. About 3-5 minutes.
Add tomato sauce and salt to the oil and onions and let the flavors combine for a few minutes.
Add butter and let it melt into the sauce. Don't mix until it is completely melted.
Simmer and stir the sauce often.
Lasagna Filling instructions
In a large bowl add the ricotta cheese, eggs, grated parmesan, parsley, salt, and pepper. Mix all of the ingredients until combined.
It is time to prepare your baking dish.
Take a small ladle or spoon and put 3 scoops of sauce on the bottom of the dish.
Next, place 3 lasagna noodles on the sauce base you just poured in the dish.
Spread a good scoop of ricotta cheese on each noodle. I prefer a thinner layer but you can go as heavy or light as you prefer.
Please a couple slices of fresh mozzarella on each noodle. That should be about 6 slices per layer.
Ladle a little sauce over the cheese layer. I prefer less rather than more.
Add the next noodle later and repeat layers ending with noodles on top.
Ladle sauce just to coat the noodles on the final layer.
Add mozzarella slices and grated parmesan cheese.
Cover the baking dish with tinfoil and bake for 45 minutes. Take the tinfoil off at the 30 minuted mark and bake uncovered for the last 15 minutes.
***Everything I do is often to taste. Don't be afraid to taste the ricotta mixture as well as the simple sauce as you go and add more salt if you desire. You can always add more. I like to add and taste as I go.