In a pan on medium heat, add oil and butter. Once it melts add chopped garlic.
Saute garlic for few seconds, till you start getting a nice aroma and then add chopped onions.
Saute onions for 1-2 minutes or till they become translucent.
Add chopped spinach [note: I cooked/thawed my frozen spinach according to instructions on the package before adding to the pan] to the pan and mix. Cook till there's no moisture left. Season with salt and pepper and set aside.
Transfer the spinach mixture to a large bowl. Add parmesan cheese, ricotta cheese to it.
Also add 1 egg, dried oregano and salt and pepper to taste.
Mix everything till well combined.
And now basically you have to make layers using 3 ingredients - marinara sauce [divide in 3 equal parts], lasagna noodles and the spinach-ricotta mixture [divide in 3 equal parts].
Start by pouring some marinara sauce on the bottom of a 13 x 9 inch pan.
Then place 3 lasagna noodles on top of it [1st layer of noodles]. I used little more than 3 noodles to cover the pan.
Cover the noodles with the spinach ricotta mixture next.
And keep making layers using spinach ricotta mix, lasagna noodles and marinara sauce. [See above for detailed pictures].
After all layers are done, top with shredded mozzarella cheese.
Coat one side of a large aluminium foil with non-stick pray and cover the pan with the foil (with the oil side facing down).
Bake at 350 F degrees for 40-45 minutes. Remove the foil and bake additional 15 minutes or till top is browned.
Take out of the oven and let the spinach lasagna cool down for 20 minutes before serving.
Cut into squares and serve immediately. I sprinkled some fresh rosemary on top.