Wash, remove stems, and cut tomatoes and onion into large chunks. Place them on the sheet pan. Sprinkle salt, black pepper, and Italian Seasoning over tomatoes and onions.

Cut the top off of the garlic head and wrap the bottom portion with aluminum foil. Place the foil packet on the corner of the sheet pan.

Bake for one hour and 15 minutes, until roasted and slightly browned. Remove from the oven and cool for 15 minutes.

Remove as many skins from roasted tomatoes as possible. I use my hands to do this once the tomatoes have cooled slightly. Discard the skins. (Don't worry if you miss some, or forget this step, as the skins will break down in the blender.

Add contents of the sheet pan to the blender. Squeeze the head of garlic and add the softened garlic to the blender. Discard the skins. Blend until well pureed, about 1 minute.

In a large saucepan, add a tablespoon of olive oil and the tomato paste. Cook over low heat until soft. Add the vegetable broth to the pan and stir to combine. Stir in granulated sugar until dissolved. Bring to a simmer.

Add the contents of the blender to the saucepan and simmer for 15 minutes, until flavors are melded together and soup it thickened. Salt and pepper to taste.