Add butter/oil and onion to a large pot over medium high heat. While the chopped onion browns, prepare the spinach puree.
For the puree, I used approximately 3 cups of spinach and about 1/2C of water. Use a high powered blender or food processor to get a puree. This yielded about 1.5 cups for me.
Pour the spinach puree into the pot and let it simmer with the onions. Then, prep the carrots.
Cook 3-4 carrots (I just used the microwave) until soft. Place in blender/food processor with a little water and blend until pureed. Yields about 1 cup.
While the carrots cook, you can chop up the mushrooms (either pulse in the processor/blender or finely chop by hand).
Add carrot puree and mushrooms to the pan.
Then, stir in the 2 jars of marinara sauce.
Let simmer for about 20 minutes (to let some of the excess water cook off) and serve or freeze for later!