Heat oil in a medium stockpot over medium heat. Add onion and garlic and sauté until onion begins to caramelize, approximately 15 minutes. Add thyme and carrot and continue to sauté for 5 additional minutes. Add tomatoes and their juice and bring to a boil over high heat. Add optional Parmesan rind, if you choose. Reduce heat and allow to simmer for 30 minutes. Season to taste with salt. Note: For pizza sauce, I purée the sauce with an immersion blender right in the pot. This sauce freezes wonderfully. I freeze leftovers in ½-cup portions.