Heat a large deep pot or Dutch oven over medium-low heat, then add the butter, olive oil, celery, carrots and onions; cook 7 to 8 minutes, or until soft. Add the garlic and crushed red pepper flakes, cook 1 minute more, until fragrant. Add tomato paste and cook an additional minute.
Add the crushed tomatoes, chicken bone broth, salt, black pepper, the cheese rind (if using) thyme and bay leaf. (I like to tie them together so they’re easy to remove later)
Cover and cook on low for 1 ½ to 2 hours, stirring occasionally until the vegetables get soft and the flavors blend, uncovering towards the end if you want it to thicken. I also add the basil during the last 30 minutes.
Remove the cheese rind, bay leaves and thyme, add the grated cheese and, using an immersion blender, blend the soup until smooth (or you can carefully do this in small batches in the blender). At this point if you want to use half and half or a splash of cream, add it, then taste for salt and black pepper, and adjust to taste. Cover and simmer low until ready to eat. Makes 7 ½ to 8 cups.
Notes
This soup relies on good quality canned tomatoes, I tested with several brands and loved it with Tuttorosso. This is also great with San Marzano tomatoes.*The amount of salt will depend on the brand of tomatoes and broth you use, and if you include the cheese rind. If it tastes flat and bland with 3/4 teaspoon, add more as canned tomatoes dull easily, salt wakes them up.Chicken bone broth brands vary in taste and salt, so taste it before you add to the soup to make sure you like it. Better Than Boullion can be used in addition to broth for more flavor, or in place of.