Preheat the oven to broil. Cut the bell peppers into 3-4 sections each, removing the stem and seeds. Lay them out on a large baking sheet, waxy-side-up. Place on the upper rack and broil until the skins are mostly black. Then add the garlic cloves to the baking sheet and broil another 1-2 minutes.
Use tongs to move the blackened peppers to a large paper grocery bag, or a large plastic zip bag. Seal the bag tightly and let the peppers steam for 15 minutes to loosen the black skin.
Meanwhile, shred the onion and carrot on a grater. Add the butter, onions, and carrots to a large deep skillet or sauce pot. Set over medium-low heat and sauté the veggies until they are very soft.
Once the roasted red peppers have steamed, pull the black waxy skin off each piece and discard. Peel the garlic cloves. Place the soft roasted peppers and garlic cloves in the blender. Puree until smooth.
Pour the pureed peppers into the skillet, and add the Italian seasoning, 1 1/2 teaspoons salt, and 1/2 teaspoon ground black pepper. Simmer for at least 10 minutes before serving.