To a high-powered blender, add the fresh tomatoes, zucchini, and tomato paste. Blend on high until the mixture is smooth and seeds / tomato skins are pulverized. If your blender gets stuck or won’t blend completely, feel free to add 1 of the 4 cups of water to the blender (but then use 3 cups of water in Step 4). Once blended, set this aside.
Before you get the stove started, I find it helpful to mince all the veggies we’ll be using ahead of time. You can do this by hand, or (my preference) use a small food processor or vegetable chopper to mince each vegetable: onions, garlic, mushrooms, carrots, celery, and bell pepper. I then keep the veggies in small bowls, so they’re ready to be added to the pot when needed!
Heat the olive oil in a large saucepan on medium high heat. Add your onions, garlic, salt, and dried basil. Stir to coat and saute for 5-7 minutes, until the onions are translucent.
Then, add the mushrooms, carrots, celery, and red bell pepper. Stir and cook another 2-3 minutes.
Add the tomato mix from the blender, 4 cups water, canned tomatoes, wine, and bouillon. Stir, cover, and bring to a simmer.
When simmering, remove the cover and stir in the spinach, brown sugar, Italian seasoning, and red pepper flakes. Turn heat to medium low and allow the sauce to simmer uncovered, stirring occasionally for about 40 minutes.
At this point, if you'd like the sauce to be smoother, use an immersion blender to blend all the ingredients together now! I like to use mine just a little, so the sauce is a nice mix of creamy and chunky.
Stir again after you blend and continue to cook, stirring and scraping the bottom periodically, for 1-3 hours or until the sauce reaches your desired consistency.
Before serving, taste and adjust the seasonings as you see fit. If it's too tart, you can add another Tbsp or two of brown sugar. I know it seems weird, but this totally balances the sauce out without making it overtly sweet!
Serve this sauce over pasta dishes, in any recipe that calls for marinara or pasta sauce, or as a dipping sauce for breadsticks or on top of pizzas and the like! See leftover instructions in the notes section below. Enjoy!